Before, we'd pass `vendor/gitaly-ruby` as artifacts to other jobs in the
pipeline but now that it's saved in cache, it can happen that the cache
doesn't have the exact versions needed by the job since its key is
fixed (i.e. currently "ruby-go-cache-v1").

This commit make sure that Gitaly dependencies are installed (with
`scripts/gitaly-test-build`) in the jobs that call
`scripts/gitaly-test-spawn`. It also makes the cache key dependent on
the `GITALY_SERVER_VERSION` file so that each time this file changes,
the cache key will change as well.

Note that we need to remove the `tmp/tests/gitaly/.ruby-bundle` file in
the `setup-test-env` job since this file prevents gems from being
installed even if `vendor/gitaly-ruby` doesn't have the correct gems.

Also, extend from `.rails-cache` in `frontend-fixtures`.
Signed-off-by: Rémy Coutable <remy@rymai.me>