refactor: impure.nix calculates localSystem for itself; avoid duplication since otherwise integrations/nix-update/default.nix would need to add that too

default.nix

@@ -2,6 +2,4 @@
 let
   sane-nix-files = import ./pkgs/by-name/sane-nix-files/package.nix { };
 in
-  import "${sane-nix-files}/impure.nix" ({
-    localSystem = builtins.currentSystem;
-  } // args)
+  import "${sane-nix-files}/impure.nix" args

impure.nix

@@ -4,7 +4,9 @@
 # race conditions or eval failures.
 #
 # see default.nix for a wrapper around this with better purity guarantees.
-{ localSystem }:
+{
+  localSystem ? builtins.currentSystem,
+}:
 let
   mkPkgs = branch: args: (
     (import ./pkgs/by-name/nixpkgs-bootstrap/${branch}.nix {}).override args