This change adds the '-modcacherw' build flag, which leaves
newly-created directories (but not the files!) in the module cache
read-write instead of making them unwritable.

Fixes #31481

Change-Id: I7c21a53dd145676627c3b51096914ce797991d99
Reviewed-on: https://go-review.googlesource.com/c/go/+/202079
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>