cmd/go: insert goroot to the hash of build cache when the packages include C files

This reverts CL 351851, which itself reverted CL 348991.

The problem with the original CL, as far as I can tell, was due to a
bug in the Go project's builder infrastructure (#33598) and not the
change itself. Once the build infrastructure is fixed, this change
can be resubmitted.

Fixes #48319
Updates #33598

Change-Id: I0fdbcc241eb2bdeb350944aad58bf58774fb591e
Reviewed-on: https://go-review.googlesource.com/c/go/+/353352


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>