Streamline GA workflows and triggers

[underchemist]

The current workflows (ci-test.yml and ci-build-push.yml) have some redundancy and can potentially be streamlined to avoid:

• overlapping event triggers in certain cases. ci-test.yml called on pull request and all push events, while ci-build-push.yml called on only pushes to main.
• tests are run by both, but in slightly different ways (ci-build-push.yml will call scripts/test with --no-cache arg)
• ci-build-push.yml will build an image more than once, can we instead reuse if tests pass?
• flak8 and mypy are called directly, ~can we instead use https://github.com/pre-commit/action to ensure outputs are consistent?~ (deprecated)

I suggest that

• Either consolidate into single workflow, or clearly separate their intended functionality e.g. tests run on pull requests and commits, while build & push are triggered only for tagged commits
• Add tag as an event trigger to support tagged releases on ghcr.io (and later to pypi per #16)

[JBurkinshaw]

This sounds like a good approach 👍