gdbus-peer: Specifically listen on 127.0.0.1

This bypasses any issues we might have with containers where IPv6 is returned by name resolution (particularly since !616 (merged)) but doesn't necessarily actually work.

This comes at a minor test-coverage cost: we don't test GDBusServer's default behaviour when told to listen on "tcp:" or "nonce-tcp:", and on systems where IPv6 is available, we don't test it. If we want to do those, we should perhaps do them in separate tests, and disable those tests when binding to ::1 doesn't work.

Mitigates: #1912 (closed)

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Maybe this will fix #1912 (closed) well enough to give us our CI back?