rust_uprev: check for files in gs:// before downloading them

I had to `--reset` a few times; downloading these files just to not
upload them took a bit, and wasn't useful. This CL adds a check so we
don't do that in the future.

BUG=chromium:1136579, chromium:1112551
TEST=Ran the script

Change-Id: I03f9f21cc07a7b16b540a5be5b4b08e105fc81a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2462926
Reviewed-by: Tiancong Wang <tcwang@google.com>
Tested-by: George Burgess <gbiv@chromium.org>
2 files changed