Allow GOCACHE=off only for operations that never actually write
anything to the cache (in which case the GOCACHE setting should not
matter at all).

Fixes #29127

Change-Id: I733d02cd2fbcf3671f5adcfb73522865d131e360
Reviewed-on: https://go-review.googlesource.com/c/153462
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>