[release-branch.go1.13] cmd/go: don't include package dir in cache key when -trimpath is set

The '-trimpath' flag tells 'go build' to trim any paths from the
output files that are tied to the current workspace or toolchain. When
this flag is set, we do not need to include the package directory in
the text hashed to construct the action ID for each package.

Updates #33772
Fixes #34326

Change-Id: I20b902d2f58019709b15864ca79aa0d9255ae707
Reviewed-on: https://go-review.googlesource.com/c/go/+/195318


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit aa680c0c)
Reviewed-on: https://go-review.googlesource.com/c/go/+/198259


Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>