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>