Add dedicated error message for musl install attempts (#9430)

Until https://github.com/astral-sh/uv/issues/6890 is fixed, it seems
nice to explain that we do not support it rather than the generic
message in https://github.com/astral-sh/uv/issues/9428

crates/uv-python/src/platform.rs

@@ -49,6 +49,10 @@ impl Libc {
             _ => Ok(Self::None),
         }
     }
+
+    pub fn is_musl(&self) -> bool {
+        matches!(self, Self::Some(target_lexicon::Environment::Musl))
+    }
 }
 
 impl FromStr for Libc {