Compress the initramfs using zstd

Fedora CoreOS has been using a zstd compressed initramfs for a while now, but it's not the default in dracut / Fedora yet.

We should consider using that for bootable containers.

See:

• https://bugzilla.redhat.com/show_bug.cgi?id=1554931
• https://github.com/coreos/fedora-coreos-config/pull/1844
• https://github.com/coreos/fedora-coreos-config/blob/testing-devel/overlay.d/05core/usr/lib/dracut/dracut.conf.d/60-coreos-zstd.conf
• fedora/ostree/sig#34 (closed)