ci: use starflow CI actions

[lengau]

• Have you followed the guidelines for contributing?
• Have you signed the CLA?
• Have you successfully run tox?

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CRAFT-3603

Depends on canonical/starflow#18

[tigarmo]

@lengau I think I'll do the review starting from the final UI; let me know if this is too unreadable:

[tigarmo]

also @lengau can you break a test so we see what a failed run looks like?

[lengau]

@tigarmo thanks for the review!

For the questions you asked:

1. I can name the step whatever I want, so I can do that. (commit incoming)
2. I don't think I can hide that output in a composite action. The only way I can see being able to do it is by putting it into a separate step in a reusable workflow. I'm not sure I'm particularly happy with the reusable workflow I came up with for this earlier though: https://github.com/canonical/starflow/pull/10/files
3. I'm trying a notice message - not sure whether it'll work, will report back.
4. While this isn't a broken test (it was broken coverage), this has the same effect: https://github.com/canonical/starbase/actions/runs/11469498939/job/31916844851

[mattculler]

nice

[upils]

LGTM except for a small comment on the uv.lock file.

[tigarmo]

great!