Add support for dynamic musl Python distributions on x86-64 Linux (#12121)
Following the upstream release and #12120, removes gating preventing
installation of the managed musl Python versions.
Of note
- The filtering of musl Python distributions has moved from the Rust
runtime to the metadata fetcher
- The filtering is now conditional on the PBS release date, removing all
old static musl distributions
- We could support the `+static` musl downloads in the future; right
now, they are deprioritized when selecting a variant
- I added test to CI which uses Alpine and installs numpy