mkosi: Do not redirect brtfs output to /dev/null
mkosi: Do not redirect brtfs output to /dev/null

When the commands fail, it is useful to see the their error output.
In the normal case they don't print anything, so it should be OK
to just remove the redirection.

UbiCastTeam/mkosi,systemd/mkosi,systemd/mkosi,UbiCastTeam/mkosi