ci: disable nilla overrides

The default overrides in nilla slow things down when evaluating parts of
a configuration - even parts that don't need them! Although it's a shame
I think that disabling them is the correct course of action for now...

packetmix/nilla.nix

···
     ];
 
     config = {
+      assertions = lib.modules.overrides.force []; # The default assertions currently force us to fetch every input to evaluate, and end up causing more trouble than they are worth. Let's skip them...
+      
       packages.allNixOSSystems = {
         systems = [ "x86_64-linux" ];