Run detect-tests and rubocop for as-if-foss pipelines

What does this MR do and why?

Run detect-tests and rubocop for as-if-foss pipelines.

Does it work?

Good that the as-if-foss pipeline is pretty clean: https://gitlab.com/gitlab-org/gitlab-foss/-/pipelines/1238813318 (We shouldn't need to run assets jobs, but that's a separate concern)

• ✅ Artifact rspec/changed_files.txt from the detect-tests job in the as-if-foss pipeline: https://gitlab.com/gitlab-org/gitlab-foss/-/jobs/6540374578/artifacts/external_file/rspec/changed_files.txt
  • Content: .gitlab/ci/as-if-foss.gitlab-ci.yml .gitlab/ci/rules.gitlab-ci.yml scripts/setup/generate-as-if-foss-env.rb spec/scripts/setup/generate_as_if_foss_env_spec.rb
• ✅ Rubocop in the as-if-foss pipeline only check the files in the changed files: https://gitlab.com/gitlab-org/gitlab-foss/-/jobs/6540374631
  • It ran fail_on_warnings bundle exec rubocop --parallel --force-exclusion .gitlab/ci/as-if-foss.gitlab-ci.yml .gitlab/ci/rules.gitlab-ci.yml scripts/setup/generate-as-if-foss-env.rb spec/scripts/setup/generate_as_if_foss_env_spec.rb and got:

    Inspecting 2 files
    ..
    2 files inspected, no offenses detected

Related to #454283 (closed)