cmd/go: use overlaid path contents in build cache

cmd/go: use overlaid path contents in build cache

When caching actions, use the overlaid file contents, because those
are the ones actually used to produce the outputs.

For #39958

Change-Id: Ia1f85b2fcf1f26e3b5be82f4d35c2726b134a36b
Reviewed-on: https://go-review.googlesource.com/c/go/+/266720
Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>