Detect musl and error for musl pbs builds (#6643)
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 before we have a python
interpreter. I changed the strategy a big to support an empty `PATH`
which we use in the tests.
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 Plan
Tested manually.
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
```
