Draft: pkg: rpm: use shadow-utils if user/group was not created using sysusers

Unfortunately, what used to work in CI (before this MR) doesn't work in COPR (permission issue in every rpm distro).

+ install -m 644 -D build_rpm/systemd/knot-resolver.sysusers /usr/lib/sysusers.d/knot-resolver.conf
install: cannot create regular file '/usr/lib/sysusers.d/knot-resolver.conf': Permission denied

Unfortunately, installing a file directly into the system is not standard practice, which is why the permissions fail. Normaly, such files are copied to the _buildroot prefix directory. Unfortunately, sysusers macro is then unable to find this file, and I don’t know why.

The only solution that worked for me: If user/group isn't created using sysusers, it is created using the old method (shadow-utils).