iputils: Increase the priority to avoid collisions with inetutils (#78408)

The ping binary will be from iputils anyway since it's installed in
/run/wrappers/bin/ (first in $PATH) and tftpd will be from iputils as
well since inetutils installs it into libexec (not in $PATH).

Therefore, there's not even a real choice here (though we should most
likely prefer iputils anyway).

Current collisions:
collision between `/nix/store/...-iputils-20190709/bin/ping' and `/nix/store/...-inetutils-1.9.4/bin/ping'
collision between `/nix/store/...-iputils-20190709/share/man/man8/tftpd.8.gz' and `/nix/store/...-inetutils-1.9.4/share/man/man8/tftpd.8.gz'