build bootable hurd image, check reproducibility and boot inside qemu

Marked as draft as we still need to fix unreproducibility in hurd.postinst https://salsa.debian.org/hurd-team/hurd/-/blob/master/debian/hurd.postinst?ref_type=heads#L77 with /var/lib/random-seed

EDIT: wait for hurd with hurd-team/hurd!4 (merged) to get uploaded to unstable

EDIT2: ready now