cmd/go: add -C flag

The -C flag is like tar -C or make -C: it changes to the named directory
early in command startup, before anything else happens.

Fixes #50332.

Change-Id: I8e4546f69044cb3a028d4d26dfba482b08cb845d
Reviewed-on: https://go-review.googlesource.com/c/go/+/421436
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>

src/cmd/go/internal/work/build.go

@@ -58,6 +58,10 @@ will be written to that directory.
 The build flags are shared by the build, clean, get, install, list, run,
 and test commands:
 
+	-C dir
+		Change to dir before running the command.
+		Any files named on the command line are interpreted after
+		changing directories.
 	-a
 		force rebuilding of packages that are already up-to-date.
 	-n
@@ -282,6 +286,7 @@ const (
 // install, list, run, and test commands.
 func AddBuildFlags(cmd *base.Command, mask BuildFlagMask) {
 	base.AddBuildFlagsNX(&cmd.Flag)
+	base.AddChdirFlag(&cmd.Flag)
 	cmd.Flag.BoolVar(&cfg.BuildA, "a", false, "")
 	cmd.Flag.IntVar(&cfg.BuildP, "p", cfg.BuildP, "")
 	if mask&OmitVFlag == 0 {