system.build: Treat as variables, make lazy

This fixes an unnecessary evaluation dependency that prevented the
custom and much appreciated primaryUser error from popping up.

Specifically:

       … while evaluating the option `system.build':

       … while evaluating definitions from `/nix/store/lc6n4bhxj9255kzfn9pnpx65583a8cgc-source/modules/environment':

       … while evaluating definitions from `/nix/store/lc6n4bhxj9255kzfn9pnpx65583a8cgc-source/modules/nix':

       … while evaluating the option `environment.darwinConfig':

       … while evaluating the option `system.primaryUserHome':

       error: expected a string but found null: null
       at /nix/store/lc6n4bhxj9255kzfn9pnpx65583a8cgc-source/modules/system/primary-user.nix:26:30:
           25|       default =
           26|         config.users.users.${config.system.primaryUser}.home or "/Users/${config.system.primaryUser}";
             |                              ^
           27|     };

While it did have some indication as to the cause, it lets the good
error message go to waste.

**Context**

`lazyAttrsOf` is the better choice when you use an attrset as individual
variables instead of in aggregate (e.g. `attrNames`, `toJSON`).

The reason is that an expression like `a.b` is strict in `a`, which
entails the evaluating the _whole_ set of attribute _names_ in `a`.
In the `attrsOf` this means evaluating all `mkIf` conditions, which
in turn also means evaluating all the regular definitions to the
smallest degree (WHNF) to determine that they're not `mkIf`s.

`lazyAttrsOf` simply assumes that all attributes aren't `mkIf false`,
and throws an error in the attribute value if necessary.
This would be a problem with `toJSON` and such, but is completely
fine when the attributes are treated as variables of a lazy program,
as is the case here.

**NixOS**

NixOS made `system.build` a submodule with a `freeformType`, allowing
the things inside of it to be declared, and for them to have niceties
like documentation and merging behavior.
nix-darwin could probably adopt this.

modules/system/default.nix

@@ -20,7 +20,7 @@ in
 
     system.build = mkOption {
       internal = true;
-      type = types.attrsOf types.unspecified;
+      type = types.lazyAttrsOf types.unspecified;
       default = {};
       description = ''
         Attribute set of derivation used to setup the system.