cmd/dist: do not use user build cache in versioned trees

cmd/dist: do not use user build cache in versioned trees

There is no guarantee that the user build cache will have
correct data if we are using a versioned build (with a VERSION file),
because that overrides the use of tool build IDs for staleness.
An earlier build might have run with a buggy compiler, and we don't
want those files lying around.

Change-Id: I831956911162ccbd0b4d943c305b3537918fe119
Reviewed-on: https://go-review.googlesource.com/c/go/+/502699
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>