Get a newer version, since newer Apple Clang (v21) in CI is stricter than the previous v17 and yield compile-errors. So let's update.
mu embeds a "vendored" subset of the libfmt package under thirdparty; but it's better to use the system-package if the user has one. So, use the system package if found or user -Duse-embedded-fmt=true is specified.
