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

crates/uv-python/src/platform.rs

@@ -126,6 +126,10 @@ impl Arch {
     pub fn family(&self) -> target_lexicon::Architecture {
         self.family
     }
+
+    pub fn is_arm(&self) -> bool {
+        matches!(self.family, target_lexicon::Architecture::Arm(_))
+    }
 }
 
 impl Display for Libc {