A custom runtime can be specified by setting the `HERMETIC_PYTHON_URL`, which
may point to a mostly arbitrary URL, i.e. one not following the same format
as python-build-standalone URLs, such e.g. `file:///__w/jax/jax/python-tsan.tgz`.
Internally, some rules_python code assumes the python-build-standalone URL
format and eventually errors.
To fix, the python version is used to determine the URL build suffix.
This also patches the rule_python versions URL generation logic to add musl
to the list of platforms it generates freethreaded runtime urls for.
Work around for https://github.com/bazel-contrib/rules_python/issues/3285, https://github.com/bazel-contrib/rules_python/issues/3286
PiperOrigin-RevId: 810132685
