[release-branch.go1.12] cmd/go/internal/cache: disable builds if GOCACHE is not an absolute path
If GOCACHE is set but is not an absolute path, we cannot build.
And GOCACHE=off also returns the error message "build cache is
disabled by GOCACHE=off".
Fixes #30493
Change-Id: I24f64bc886599ca0acd757acada4714aebe4d3ae
Reviewed-on: https://go-review.googlesource.com/c/164200
Run-TryBot: Baokun Lee <nototon@gmail.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit
13d24b685a6d7b05a249f85be91c390f5595f745)
Reviewed-on: https://go-review.googlesource.com/c/164717
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>