copy_rust_bootstrap: use gsutil

gsutil.py is no longer supported in the chroot:
https://crrev.com/860a3a19d64f00a3876f23e2465c6bc241fb97ce

BUG=b:278606581
TEST=Ran the script

Change-Id: Iffda99cb85207b98b8f4d6d57e8045fce5314a33
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/4437270
Reviewed-by: Bob Haarman <inglorion@chromium.org>
Commit-Queue: George Burgess <gbiv@chromium.org>
Tested-by: George Burgess <gbiv@chromium.org>
1 file changed