In test change group of directory again

This has no effect on the test itself but the code may be used as an example where it does matter.
Since the systemd unit runs with DynamicUser=true, the group ID of the directory may change over time, so reset it.
This reverts part of b913fc2.

nix/tests/NixOS-integration-test.nix

@@ -43,6 +43,7 @@ self:
       script = ''
         mkdir --verbose -p '/var/lib/bind/zones/dyn/'
         chmod 775 '/var/lib/bind/zones/dyn/'
+        chgrp zonegen '/var/lib/bind/zones/dyn/'
 
         # Create an initial file for zonewatch and BIND to read
         (set -o noclobber;>'/var/lib/bind/zones/dyn/example.org.zone'||true) &>/dev/null