As described in #4242, we're currently incorrectly downloading glibc python-build-standalone on musl target, but we also can't fix this by using musl python-build-standalone on musl targets since the musl builds are effectively broken.

We reintroduce the libc detection previously removed in #2381, using it to detect which libc is the current one.

For simplicity, i've decided to just filter out the musl python-build-standalone archives from the list of available archive, given this is temporary. This means we show the same error message as if we don't have a build for the platform. We could also add a dedicated error message for musl.

Fixes #4242

## Test strategy

On my ubuntu host, python downloads continue to pass:
```
target/x86_64-unknown-linux-musl/debug/uv python install
```

On alpine, we fail:
```
$ docker run -it --rm -v .:/io alpine /io/target/x86_64-unknown-linux-musl/debug/uv python install
  Searching for Python installations
  error: No download found for request: cpython-any-linux-x86_64-musl
```