For devel builds we use current date as ISO version. Allow to provide
specific value, without marking the ISO as final.

This isn't exactly "last modification time of source", as the source for
the final ISO include also all rpm packages from other components (and
from Fedora), which may be newer than installer-qubes-os itself. But in
practice it shouldn't be a problem as most of them will be only included
as files without relying on any specific timestamps inside.

Package root filesystem directly as squashfs image. The most important
gain is getting rid of greatly unreproducible ext4 image creation. But
also it makes the output image slightly smaller, and the build faster.
Also, dmsquash-live dracut module is not needed anymore, which reduce
initramfs size.

Since moving to squashfs + overlay fs, it isn't needed anymore. And size
of initrd for efi is critical.

…image

Remove various caches not really needed.
Sort group files and packages list.
Strip timestamps from gconf file.
Clamp mtime in fontconfig cache to SOURCE_DATE_EPOCH.

… 5.0-rc1