kola/testiso: make volume ID test arch-independent

The actual backing device used for the live ISO is arch-dependent. For
example on aarch64, it actually just looks like another virtio disk
(e.g. `/dev/vdb`).

But we know that it must be mounted at `/run/media/iso`, so key off of
that instead to find the backing device.

Fixes 06da492 ("kola/testiso: test volume ID has OS name prefix").

Author: jlebon

mantle/cmd/kola/testiso.go

@@ -272,10 +272,12 @@ var verifyIsoVolumeId = `[Unit]
 Description=Verify ISO Volume ID
 OnFailure=emergency.target
 OnFailureJobMode=isolate
+RequiresMountsFor=/run/media/iso
 [Service]
 Type=oneshot
 RemainAfterExit=yes
-ExecStart=bash -c "[[ $(lsblk -no LABEL /dev/sr0) == %s-* ]]"
+# the backing device name is arch-dependent, but we know it's mounted on /run/media/iso
+ExecStart=bash -c "[[ $(lsblk -no LABEL $(findmnt -no SOURCE /run/media/iso)) == %s-* ]]"
 [Install]
 RequiredBy=coreos-installer.target`