Refactor and cleanup FTL build workflows

[PromoFaux]

What does this PR aim to accomplish?:

I had initially wanted just to add in an additional condition for tagging nightly, however while I was there I took some inspiration from the build process of the docker-pi-hole image

• tag nightly builds on both schedule and push to master branch
• tag latest and exact tag name on "release" types
• only set up QEMU for relevant platform
• No need for QEMU on the merge job
• Set riscv64 runner to arm variant
• utilise YAML anchors to reduce duplication
• separate publish and test jobs to make it easier to follow and to simplify the logic of when it should run

---

By submitting this pull request, I confirm the following:

1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code and I have tested my changes.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

---

• I have read the above and my PR is ready for review. Check this box to confirm

[dschaper]

Looks like the security scan bot has left some comments and suggestions for limiting access.

[PromoFaux]

They've been addressed :)

[yubiuser]

@PromoFaux There are merge conflicts

[yubiuser]

I've been thinking about using https://github.com/docker/github-builder for those workflows as well, but the retry action (https://github.com/nick-fields/retry) likely won't work with the reusable workflow approach.
If we switch, we gain much easier workflows, but at the expense of needing to manually re-start failed runs.

[PromoFaux]

Closing in favour of #152 - will push a change to that