If `setup-julia` moves to using juliaup, cache juliaup dir

[IanButterworth]

Using juliaup within setup-julia might make sense https://github.com/julia-actions/setup-julia/issues/172

If so, given juliaup saves julia versions in .julia/juliaup by default, it would mean we could cache julia versions.

This could potentially speed up Windows installation significantly, as currently it takes 1 min due to slow extraction of highly compressed installers. The cache extraction seems to be quite fast.

Though, 1.9.4 install is ~400 MB on disk, so 10x bigger than caches I've tested.