Actions: Run CI only for PRs from forks. (#489)

.github/workflows/linux.yml

@@ -6,6 +6,10 @@ on:
 
 jobs:
   build:
+    # We want to run on external PRs, but not on our own internal PRs as they'll be run
+    # by the push to the branch.
+    if: github.event_name == 'push' || github.event.pull_request.head.repo.full_name != github.repository
+
     runs-on: ubuntu-latest
     steps:
       - name: Checkout