systems: allow passing in string for cross/localSystem

This makes things a little bit more convenient. Just pass in like:

$ nix-build ’<nixpkgs>’ -A hello --argstr localSystem x86_64-linux --argstr crossSystem aarch64-linux

lib/systems/default.nix

@@ -14,7 +14,9 @@ rec {
   # `parsed` is inferred from args, both because there are two options with one
   # clearly prefered, and to prevent cycles. A simpler fixed point where the RHS
   # always just used `final.*` would fail on both counts.
-  elaborate = args: let
+  elaborate = args': let
+    args = if lib.isString args' then { system = args'; }
+           else args';
     final = {
       # Prefer to parse `config` as it is strictly more informative.
       parsed = parse.mkSystemFromString (if args ? config then args.config else args.system);