cmd/go: use 'gcloud storage cp' instead of 'gsutil cp'

In some misguided attempt at "cleanup", Google Cloud has
decided to retire 'gsutil' in favor of 'gcloud storage' instead
of leaving an entirely backwards-compatible wrapper so
that client scripts and muscle memory keep working.

In addition to breaking customers this way, they are also
sending AI bots around "cleaning up" old usages with scary
warnings that maybe the changes will break your entire world.
This is even more misguided, of course, and resulted in us
receiving CL 748820 (originally GitHub PR #77787)
and then me receiving a private email asking for it to be merged.

It was easier to recreate the 3-line CL myself than to
enumerate everything that was wrong with that CL's
commit message.

I hope that only Google teams are being subjected to this.

Fixes #77787.

Change-Id: I624ad4aff7f488b191e195b72e564dbd78440883
Reviewed-on: https://go-review.googlesource.com/c/go/+/748900
TryBot-Bypass: Russ Cox <rsc@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

src/cmd/go/testdata/script/mod_get_pseudo_other_branch.txt

@@ -64,4 +64,4 @@ git checkout master
 git merge b -m merge
 
 zip -r ../tagtests.zip .
-gsutil cp ../tagtests.zip gs://vcs-test/git/tagtests.zip
+gcloud storage cp ../tagtests.zip gs://vcs-test/git/tagtests.zip

src/cmd/go/testdata/script/mod_get_pseudo_prefix.txt

@@ -61,4 +61,4 @@ git add status
 git commit -m 'after last tag'
 
 zip -r ../prefixtagtests.zip .
-gsutil cp ../prefixtagtests.zip gs://vcs-test/git/prefixtagtests.zip
+gcloud storage cp ../prefixtagtests.zip gs://vcs-test/git/prefixtagtests.zip

src/cmd/go/testdata/script/mod_retract_pseudo_base.txt

@@ -59,4 +59,4 @@ git mv p.go q.go
 git commit -m 'trivial change'
 
 zip -r ../retract-pseudo.zip .
-gsutil cp ../retract-pseudo.zip gs://vcs-test/git/retract-pseudo.zip
+gcloud storage cp ../retract-pseudo.zip gs://vcs-test/git/retract-pseudo.zip