With https://github.com/msys2/msys2-autobuild/commit/dc632d99340da8b204fa2d7
we can now override COMPRESSZST in autobuild, to use higher than default
compression when building packages for the repo.
This means we can revert the default COMPRESSZST back to the default
zstd compression level (3) and make packaging faster for development/CI
etc.