mkosi: exclude all cache and output directories from the copied build sources
mkosi: exclude all cache and output directories from the copied build sources

We should exclude mkosi.builddir/, mkosi.output/, mkosi.cache/ when
preparing the build image, as they aren't really part of the sources,
but contain artifacts of previous mkosi invocations.

UbiCastTeam/mkosi,UbiCastTeam/mkosi,systemd/mkosi,systemd/mkosi