fix(toolchains): add musl list of freethreaded runtimes (workspace) (#3310)

The musl entry got lost as part of a refactoring. Add it back to the
platforms
that generate freethreaded variants.

Fixes https://github.com/bazel-contrib/rules_python/issues/3286

python/versions.bzl

diff --git a/python/versions.bzl b/python/versions.bzl
index 34cffc5..1b20290 100644
--- a/python/versions.bzl
+++ b/python/versions.bzl
@@ -1065,6 +1065,7 @@
                     "x86_64-apple-darwin": "pgo+lto",
                     "x86_64-pc-windows-msvc": "pgo",
                     "x86_64-unknown-linux-gnu": "pgo+lto",
+                    "x86_64-unknown-linux-musl": "pgo+lto",
                 }[p],
             )
         else: