nixpkgs module: Clean up platform options

- `localSystem` is added, it strictly supercedes system

 - `crossSystem`'s description mentions `localSystem` (and vice versa).

 - No more weird special casing I don't even understand

TEMP

nixos/modules/virtualisation/containers.nix

@@ -112,7 +112,7 @@ let
 
       # If the host is 64-bit and the container is 32-bit, add a
       # --personality flag.
-      ${optionalString (config.nixpkgs.system == "x86_64-linux") ''
+      ${optionalString (config.nixpkgs.localSystem.system == "x86_64-linux") ''
         if [ "$(< ''${SYSTEM_PATH:-/nix/var/nix/profiles/per-container/$INSTANCE/system}/system)" = i686-linux ]; then
           extraFlags+=" --personality=x86"
         fi
@@ -255,7 +255,7 @@ let
   };
 
 
-  system = config.nixpkgs.system;
+  system = config.nixpkgs.localSystem.system;
 
   bindMountOpts = { name, config, ... }: {