Work around download-artifact behavioral change for coverage

[TheJulianJES]

Proposed change

Works around an issue introduced by download-artifact to v5, where download behavior is different depending on how many artifacts are downloaded. Now, a directory isn't created for a single artifact, but only done for multiple ones.

To work around that, we now always download the coverage artifacts in a coverage directory, which we then scan using find for all coverage reports. This isn't elegant, but should be fine for our use-case.

Additional information

For us, this was introduced by:

• Update GitHub actions, use setup-uv action #31 (only fails for repos where we are testing a single Python version only, like zigpy-ota, hence my sample run for zha-quirks didn't catch this)

More background complaints on this change:

• fix: inconsistent path behavior for single artifact downloads by ID actions/download-artifact#416 (see comments)
• [docs] Release notes for v5 are misleading, behavior changes in more cases actions/download-artifact#419
• [bug] Download of all artifacts does not create directory with artifact name when there is only one artifact actions/download-artifact#426

[TheJulianJES]

This pattern change isn't really relevant for this PR, but I've included it anyway.
We only upload coverage artifacts at the moment, but I think it's nice to make sure we explicitly only download coverage artifacts here, and nothing else. E.g. coverage-3.11.14 and coverage-3.12.12

[TheJulianJES]

I'll merge this for now to unbreak some things. This was tested with a matrix with multiple Python versions and on one with just one Python version.

IMO, this is a really bad decision on the download-artifact, especially since there isn't an easy workaround.

All other workarounds that I saw were more complex than this, but we get away with this, as we don't need to care about the exact paths the coverage reports are saved in. We just need the files.