[GHA] Support running on hosts without pre-configured python 3.9

[mhegazy]

Add a new GH Action to setup python using uv. Also adding explicit calls to setup uv where we are executing python scripts.

Downstream CI run with an image not including python (thebrowsercompany/swift-build#302):
https://github.com/thebrowsercompany/swift-build/actions/runs/15982191821

[compnerd]

I would prefer avoiding uv. This is not used in the Swift.org builds, and diverging the builds further makes it harder to maintain. We already try to synchronise the build between build.ps1 and GHA, adding a diff on top is more complicated. Why can we not just fetch python if we want to use our own copy?

[mhegazy]

UV is doing that for us. we are still using python to run commands. what this action does is download a specific python version (specified in pyproject.toml), and sets it in the path. we could switch to a custom action, but it will do more or less the same, find architecture, find the right package to download, and then install/expand and set in the path.

[compnerd]

Yes, I realize what uv is being used for; I think that using actions/setup-python is better for that purpose. This introduces a third party dependency (and with the desire to eventually have this repository be merged into swiftlang, that complicates things further). I would rather use GitHub actions whenever there is an alternative.

[mhegazy]

@compnerd reverted back to github's actions/setup-python

[mhegazy]

Please hold off on merging this. i need to get a successful downstream run first.

[mhegazy]

@compnerd this should be updated now. downstream change passed CI on cirun and githhub: thebrowsercompany/swift-build#302

[mhegazy]

@compnerd one more iteration. using 3.9 now. downstream PR runs are passing at thebrowsercompany/swift-build#302

[compnerd]

This seems reasonable to me. One thing that we should consider is hoisting this to a global value. I'm fine with that being a follow up or done here.

[compnerd]

Discussed offline, the global value conversion can be done as a follow up.