cmd/go: add -d flag to mod_get_test

'go get all' was run in this test without -d. This caused some std
packages to be reinstalled if the test is run in a slightly different
configuration than make.bash was run. run.bash would fail in some
situations because of this. Nothing in the cmd/go tests should modify
installed std or cmd packages.

Updates #35459

Change-Id: Idd259a27d55502923b7fc54f361a77f0ac11eea2
Reviewed-on: https://go-review.googlesource.com/c/go/+/215721


Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>