dns: change rc-manager value with systemd-resolved
On Fedora and RHEL9+, rc-manager is "auto" by default, which doesn't touch resolv.conf when dns=systemd-resolved; we also want to test NM writing 127.0.0.53 to resolv.conf if needed (i.e. when resolv.conf is a regular file), so change the value of rc-manager to symlink.
Fixes-test: dns_resolved_two_default Fixes-test: dns_resolved_dnssec_opts