build: Bundle Fmt again

build: Bundle Fmt again

Fmt is currently the only required C++ dependency that isn't bundled.
Since the Fmt API is still evolving, it's convenient for Ccache users
that lack a proper Fmt version locally to be able to build without
having to download Fmt. Thus, bundle Fmt again for now. (When we can
bump compiler requirements to versions that implement std::format
properly in the future we will switch to std::format instead.)

Closes #1470.