cmd/go: fix handling of builtin packages in WriteCoverMetaFilesFile

In certain unusual situations we can wind up with a build action for a
dummy (built-in) package as a dependency for the writeCoverMeta
pseudo-action generated when -coverpkg is in effect; this was causing
a panic in WriteCoverMetaFilesFile when it discovered a predecessor
whose Mode field was not "build". Update the code that constructs deps
for writeCoverMeta action to skip dummy builds.

Fixes #67953.

Change-Id: If747aeb9bae061c84290d1e10f6ea7abb0828aca
Reviewed-on: https://go-review.googlesource.com/c/go/+/592202
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>

src/cmd/go/internal/test/test.go

@@ -1067,7 +1067,7 @@ func builderTest(b *work.Builder, ctx context.Context, pkgOpts load.PackageOpts,
 			Package:    p,
 			IgnoreFail: true, // run (prepare output) even if build failed
 		}
-		if writeCoverMetaAct != nil {
+		if writeCoverMetaAct != nil && build.Actor != nil {
 			// There is no real "run" for this package (since there
 			// are no tests), but if coverage is turned on, we can
 			// collect coverage data for the code in the package by

src/cmd/go/testdata/script/test_n_cover_std.txt

@@ -0,0 +1,7 @@
+# Issue 67953: test to make sure that the go commands static coverage
+# meta-data handling code handles pseudo-packages (ex: "unsafe") properly.
+
+[short] skip
+
+cd $GOROOT/src
+go test -vet=off -p=1 -n -coverpkg=internal/coverage/decodecounter internal/coverage/decodecounter sync unsafe