cmd/go: use os.UserCacheDir for the default GOCACHE
This piece of code predates the addition of os.UserCacheDir, and it
looks like os.UserCacheDir was based on this piece of code.
The two behaved exactly the same, minus cmd/go's addition of AppData for
Windows XP in CL 87675. However, Go 1.11 dropped support for Windows XP,
so we can safely ignore that change now.
The only tweaks necessary are to return "off" if an error is
encountered, and to disable warnings if we're using "/.cache".
Change-Id: Ia00577d4575ce4870f7fb103eafaa4d2b630743e
Reviewed-on: https://go-review.googlesource.com/c/141538
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org> Reviewed-by: Bryan C. Mills <bcmills@google.com>