Review workflows to mock env-vars including external env-file option

[pcrespov]

Follow-Up Work Required from #7919

To complete the unification:

• ✅ Standardize fixture names used for mocking environment variables
• ✅ Harmonize mocking workflows, which currently vary:
  • Some use .env-devel filtered via docker-compose.yml’s environment section
  • Others rely on different or inconsistent methods
• Unify usage of external_envfile_dict: will allow to run tests against configurations in deploys
  • Refactor so that it has to be explicitly enabled in the test that we want to use it, instead of as a generic fixture (which might have unexpected and bad consequences)

---

This can be a very DANGEROUS feature and I would like to be sure that it is clear with every dev that this can affect the deployments depending on what test(s) are being run. Otherwise I like that it is made simpler. but ideally we should prevent tests that are not only read-only...

Originally posted by @sanderegg in #7919 (review)