mkosi: rework how we reenable kernel-install snippets

mkosi: rework how we reenable kernel-install snippets

The old code doesn't really work: we can't store the list of snippets we
masked in a global variable, since we run the whole build process
multiple times and data from the first run might confuse follow-up runs
and really should not.

This reworks the logic so that we keep track precisely which snippets
are masked and return this from disable_kernel_install(). We then pass
that back into reenable_kernel_install() to undo the effect later on.
This way the information never leaks into later runs.