Fallback to system uv if Python interface isn't available

[desertaxle]

Adds a fallback to the system uv if the Python interface isn't available, which applies to the Conda distribution of uv.

[codspeed-hq]

CodSpeed Performance Report

Merging #17320 will not alter performance

_{Comparing uv-fallback (a449b13) with main (dc25f37)}

Summary

✅ 2 untouched benchmarks

[cicdw]

clean

[zanieb]

Our Python module is missing in the conda build? If so, that's news to me and we should probably fix it. Would that be like, from conda-forge or Anaconda?

[cicdw]

@zanieb there's both uv and python-uv on conda-forge; uv does not have the Python build but python-uv does so this was just a confusion for us in our feedstock recipe. We still merged this PR just in case other users make the same mistake somehow.

[zanieb]

Thanks for clarifying — that makes sense.