Change void CI image to musl

Needs https://github.com/actions/runner/issues/3852 to be resolved first.

See https://github.com/sagemath/sage/pull/40089#issuecomment-3146906523 and https://github.com/sagemath/sage/pull/40089/commits/a73ed68dc839c750fae54cb0ed714e1911f4224d for context.