distro/rhel84: use a random uuid for XFS partition

Imagine this situation: You have a RHEL system booted from an image produced
by osbuild-composer. On this system, you want to use osbuild-composer to
create another image of RHEL.

However, there's currently something funny with partitions:

All RHEL images built by osbuild-composer contain a root xfs partition. The
interesting bit is that they all share the same xfs partition UUID. This might
sound like a good thing for reproducibility but it has a quirk.

The issue appears when osbuild runs the qemu assembler: it needs to mount all
partitions of the future image to copy the OS tree into it.

Imagine that osbuild-composer is running on a system booted from an imaged
produced by osbuild-composer. This means that its root xfs partition has this
uuid:

efe8afea-c0a8-45dc-8e6e-499279f6fa5d

When osbuild-composer builds an image on this system, it runs osbuild that
runs the qemu assembler at some point. As I said previously, it will mount
all partitions of the future image. That means that it will also try to
mount the root xfs partition with this uuid:

efe8afea-c0a8-45dc-8e6e-499279f6fa5d

Do you remember this one? Yeah, it's the same one as before. However, the xfs
kernel driver doesn't like that. It contains a global table[1] of all xfs
partitions that forbids to mount 2 xfs partitions with the same uuid.

I mean... uuids are meant to be unique, right?

This commit changes the way we build RHEL 8.4 images: Each one now has a
unique uuid. It's now literally a unique universally unique identifier. haha

[1]: a349e4c659/fs/xfs/xfs_mount.c (L51)

cmd/osbuild-store-dump/main.go

@@ -26,7 +26,7 @@ func getManifest(bp blueprint.Blueprint, t distro.ImageType, a distro.Arch, d di
 	if err != nil {
 		panic(err)
 	}
-	manifest, err := t.Manifest(bp.Customizations, distro.ImageOptions{}, repos, pkgs, buildPkgs)
+	manifest, err := t.Manifest(bp.Customizations, distro.ImageOptions{}, repos, pkgs, buildPkgs, 0)
 	if err != nil {
 		panic(err)
 	}