Remove DB dependency from `setup-test-env` job

The artifacts needed from setup-test-env job are tmp/tests, config/secrets.yml, vendor/gitaly-ruby. These should not require any DB dependencies (Postgres & Redis).

The setup-test-env job currently does a full DB migration as part of .default-before_script, which takes about 1-2 minutes.

Since setup-test-env is in the critical path of gitlab:assets:compile * jobs (one of the longest running jobs), reducing the duration of setup-test-env would have direct impact on the total pipeline duration.