Reliably find uv binary

[agriyakhetarpal]

I made our uv helper match what cibuildwheel does to find uv. There's not much of a difference, but I've frequently run into issues with this when developing locally on my macOS machine and it might have been due to the functools cache for all we know.

Also, complies wth PEP 639 and fixes our licensing metadata.

[ryanking13]

Thanks. So one difference this makes is that when there is uv installed, now we will always use uv, while previously, we used uv only when the pyodide entrypoint was invoked with uv (therefore the UV env variable is set).

I think it should be okay, but it would be worth to mention.

[agriyakhetarpal]

Yes, correct. The errors I was facing were of the type where the cross-build packages were trying to be installed with pip in a uv-managed environment, but in such venvs the pip entry point does not exist deliberately and only uv pip works. I think it also makes expectations clearer, since we would expect that someone installing pyodide-build[uv] would want to use uv.