cmd/go/internal/cache: save more data from DefaultDir

cmd/go/internal/cache: save more data from DefaultDir

cmd/go/main.go sets GOCACHE explicitly, so if we don't save some
metadata about how DefaultDir arrived at its answer we will be unable
to reconstruct it later.

Fixes #29243

Change-Id: Ic8bb859ab045a29c91f6a4527e65aedabf874d53
Reviewed-on: https://go-review.googlesource.com/c/154309
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>