ci: Add opensuse-leap 15.4, skip TLS on 15.3

lcitool recently added support for Leap 15.4; meanwhile, Leap 15.3 has
a version of nbdkit that tries to use @nbdkit,SYSTEM as the TLS
priority which then crashes in gnutls.  Explicitly testing our
--without-gnutls configure coverage doesn't hurt either.

Thanks: Daniel P. Berrangé <berrange@redhat.com>