infra/image/shdefaults: Add capability SYS_ADMIN for systemd 258

Fedora 43 and 44 switched to systemd 258 rc4. The dbus-broker service
of systemd 258 does not start any more without enabling the capability
SYS_ADMIN.

The capabilities AUDIT_WRITE, SETUID and SETGID should be enough, but
they are not sufficient to be able to start the service.

With final systemd 258 it should be tested if the capability can be
removed again.