CI: Speed up static checks by checking only changed files

- file_format, header_guards and clang-format benefit from this short list.
- dotnet-format, Python and JS checks don't, but they're only relevant for
  PRs changing a specific set of files, so we skip them when those files
  aren't modified.

The logic to get changed files only works reliably for:
- Pull request events
- Non-force pushed push events

So when force pushing a branch in your fork, or creating a new branch,
it will still scan all files as fallback.

Upgraded CI runner to Ubuntu 22.04 so we get clang-format 14 out of the box,
so we don't need to install a custom version (saves ~15 s). We also cache
the APT dependencies to speed up the build and avoid flaky Ubuntu/Microsoft
repos.

.github/workflows/runner.yml

@@ -7,7 +7,7 @@ concurrency:
 
 jobs:
   static-checks:
-    name: 📊 Static
+    name: 📊 Static checks
     uses: ./.github/workflows/static_checks.yml
 
   android-build:

.github/workflows/static_checks.yml

@@ -8,38 +8,58 @@ concurrency:
 
 jobs:
   static-checks:
-    name: Static Checks (clang-format, black format, file format, documentation checks)
-    runs-on: ubuntu-20.04
+    name: Code style, file formatting, and docs
+    runs-on: ubuntu-22.04
     steps:
       - name: Checkout
         uses: actions/checkout@v3
+        with:
+          fetch-depth: 2
 
-      - name: Install dependencies
+      - name: Install APT dependencies
+        uses: awalsh128/cache-apt-pkgs-action@latest
+        with:
+          packages: dos2unix libxml2-utils moreutils
+
+      - name: Install Python dependencies and general setup
         run: |
-          sudo rm -f /etc/apt/sources.list.d/microsoft-prod.list
-          wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
-          sudo apt-add-repository "deb http://apt.llvm.org/focal/ llvm-toolchain-focal-15 main"
-          sudo apt-get install -qq dos2unix clang-format-15 libxml2-utils python3-pip moreutils
-          sudo update-alternatives --remove-all clang-format || true
-          sudo update-alternatives --install /usr/bin/clang-format clang-format /usr/bin/clang-format-15 100
-          sudo pip3 install black==22.3.0 pytest==7.1.2 mypy==0.971
+          pip3 install black==22.3.0 pytest==7.1.2 mypy==0.971
           git config diff.wsErrorHighlight all
 
+      - name: Get changed files
+        id: changed-files
+        run: |
+          if [ "${{ github.event_name }}" == "pull_request" ]; then
+            files=$(git diff-tree --no-commit-id --name-only -r ${{ github.event.pull_request.base.sha }}..${{ github.event.pull_request.head.sha }} 2> /dev/null)
+          elif [ "${{ github.event_name }}" == "push" -a "${{ github.event.forced }}" == "false" -a "${{ github.event.created }}" == "false" ]; then
+            files=$(git diff-tree --no-commit-id --name-only -r ${{ github.event.before }}..${{ github.event.after }} 2> /dev/null)
+          fi
+          echo "$files" >> changed.txt
+          cat changed.txt
+
       - name: File formatting checks (file_format.sh)
         run: |
-          bash ./misc/scripts/file_format.sh
+          bash ./misc/scripts/file_format.sh changed.txt
 
       - name: Header guards formatting checks (header_guards.sh)
         run: |
-          bash ./misc/scripts/header_guards.sh
+          bash ./misc/scripts/header_guards.sh changed.txt
 
       - name: Python style checks via black (black_format.sh)
         run: |
-          bash ./misc/scripts/black_format.sh
+          if grep -qE '*\.py|SConstruct|SCsub' changed.txt || [ ! -s changed.txt ]; then
+            bash ./misc/scripts/black_format.sh
+          else
+            echo "Skipping Python formatting as no Python files were changed."
+          fi
 
       - name: Python scripts static analysis (mypy_check.sh)
         run: |
-          bash ./misc/scripts/mypy_check.sh
+          if grep -qE '*\.py|SConstruct|SCsub' changed.txt || [ ! -s changed.txt ]; then
+            bash ./misc/scripts/mypy_check.sh
+          else
+            echo "Skipping Python static analysis as no Python files were changed."
+          fi
 
       - name: Python builders checks via pytest (pytest_builders.sh)
         run: |
@@ -47,10 +67,14 @@ jobs:
 
       - name: JavaScript style and documentation checks via ESLint and JSDoc
         run: |
-          cd platform/web
-          npm ci
-          npm run lint
-          npm run docs -- -d dry-run
+          if grep -q "platform/web" changed.txt || [ ! -s changed.txt ]; then
+            cd platform/web
+            npm ci
+            npm run lint
+            npm run docs -- -d dry-run
+          else
+            echo "Skipping JavaScript formatting as no Web/JS files were changed."
+          fi
 
       - name: Class reference schema checks
         run: |
@@ -62,11 +86,16 @@ jobs:
 
       - name: Style checks via clang-format (clang_format.sh)
         run: |
-          bash ./misc/scripts/clang_format.sh
+          clang-format --version
+          bash ./misc/scripts/clang_format.sh changed.txt
 
       - name: Style checks via dotnet format (dotnet_format.sh)
         run: |
-          bash ./misc/scripts/dotnet_format.sh
+          if grep -q "modules/mono" changed.txt || [ ! -s changed.txt ]; then
+            bash ./misc/scripts/dotnet_format.sh
+          else
+            echo "Skipping dotnet format as no C# files were changed."
+          fi
 
       - name: Spell checks via codespell
         uses: codespell-project/actions-codespell@v1