fix: always expose uv in PATH

[mayeut]

This PR changes the way uv is used in cibuildwheel for direct invocations.
At the moment, there's a mixed usage of direct invocations (assumed in PATH) & find_uv() path invocations.
#2690 has been opened to always use find_uv() but this is not really compatible with build unless it's installed with the uv extra.
In order not to install build's uv extra, find_uv is updated to symlink/copy uv in a temporary directory. This temporary directory is added to build/test virtual environments PATH which removes the need to install build's uv extra when using cibuildwheel's uv extra.

close #2690
close #2673

[mhsmith]

The Android change looks OK to me, but I don't know enough to comment on the larger question of this PR vs #2690.

[henryiii]

Build should behave similarly, using a uv in the path if there's no uv package installed?

https://github.com/pypa/build/blob/ed9c379f51abca7c0c0ae20bee66aa548d597dc3/src/build/env.py#L305-L307

[mayeut]

Build should behave similarly, using a uv in the path if there's no uv package installed?

If cibuildwheel's uv extra is not exposed in the PATH (as proposed in #2673) then build won't find uv unless it was installed with its own uv extra. See https://github.com/mayeut/cibuildwheel/actions/runs/20258197705/job/58164557217 for a failure when #2673 is applied on Windows which does not install build's uv extra.

I see you opened an issue in build and that's probably a good thing which should be applied ecosystem wide and would allow for an easier resolution.

[joerick]

Hmm. All this is starting to feel a little messy/hacky. I'm thinking there's another option here, where we say that if the build-frontend uses uv, we require the caller of cibuildwheel to ensure that uv is somewhere on PATH (and bail if it's not found). So, we don't even attempt to look in the python import path for it. That way we'll be sure that build can use it too.

In the case of the github action, we could just use astral/setup-uv conditionally, and keep the direct invocation there?

As I see it, the cibuildwheel[uv] extra would continue to work in this setup, assuming that users have the virtualenv active when they call cibuildwheel (this must be the most common setup, I'd expect).