ci: remove CI ENV & disable fail-fast

[georglauterbach]

Description

1. BATS will actaully correctly determine that it is running in a CI environment, so we can omit the CI variable (not needed at all) (see a run in a fork if mine: https://github.com/georglauterbach/docker-mailserver/actions/runs/4103019817/jobs/7076744395)
2. When a current test is failing, all other parallel jobs are stopped by GH actions due to their fail-fast approach. While this is nice for many use cases, it's not in our case - so I disabled it. Now, all tests are running to completion.

Type of change

• Improvement (non-breaking change that does improve existing functionality)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (README.md or the documentation under docs/)
• If necessary I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes