Add --die-with-parent when invoking bubblewrap

Add --die-with-parent when invoking bubblewrap

rpm blocks most signals when doing a transaction. dnf doesn't block
anything. The end result is that when we do ctrl+c when dnf is running
the rpm transaction, dnf gets interrupted and exits, which means mkosi
exits and tries to remove the workspace directory on which rpm is still
operating, causing all kinds of nastiness.

Because we don't care about transaction safety since we're operating on
a chroot, let's add --die-with-parent when running bubblewrap. This makes
bubblewrap ensure cleanup of all child processes underneath it, and
because rpm can't ignore SIGKILL, it also cleans up rpm properly, fixing
the issue.