ci: refactored sedfile & used _send_mail where possible

[georglauterbach]

Description

Obvious. Review commit by commit should be easy.

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

[georglauterbach]

Left for refactoring now:

• serial/mail_with_ldap.bats
• serial/permit_docker.bats
• serial/test_helper.bats
• serial/tests.bats

I'd also love to see the ARM64 build being cached :D

[polarathene]

Thanks for tackling this ❤️

I'd also love to see the ARM64 build being cached :D

If I can find the time to, I'll look into updating the CI to handle that. Presently juggling a few different workloads 😅