nixos: Make system.build a lazyAttrsOf unspecified

Legacy types.attrs has really bad merging behavior and does not
support priorities.

f build

nixos/modules/system/activation/top-level.nix

···
     system.build = mkOption {
       internal = true;
       default = {};
-      type = types.attrs;
+      type = types.lazyAttrsOf types.unspecified;
       description = ''
         Attribute set of derivations used to setup the system.
       '';