[release-branch.go1.14] cmd/go: include the go language version in cache keys

Fixes #37822
Updates #37804

Change-Id: I4381dc5c58cfd467506d3d73fbd19c2c7257338e
Reviewed-on: https://go-review.googlesource.com/c/go/+/223139


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
(cherry picked from commit feea3f165770025b045c6dd46747b1debdaf348e)
Reviewed-on: https://go-review.googlesource.com/c/go/+/223141


Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>