Check for existence of gitaly-ruby Gemfile

This will make it clearer why a gitaly-ruby bundle check failed. Currently there is an implicit assumption that jobs in the test stage will run within 24 hours of the setup-test-env step running. However, if a test runs after that time frame, the CI artifacts will have been destroyed, and the tmp/tests directory won't contain the right data.

This happened in https://gitlab.com/gitlab-org/gitlab/-/jobs/371020233. The gitaly-ruby bundle check quietly failed because tmp/tests/gitaly-ruby/Gemfile didn't exist. At first, we thought bundle check failed because of a stale cache or a missing Gem.