Deduplicate some filterSource invocations

This version should have more conventional regexes that work across many
platforms and regex engines. This is an issue because up until Nix 1.11,
Nix called out to the libc regex matcher, which behaved differently on
Darwin and Linux. And in Nix 1.12, we're moving to std::regex which will
also behave differently here.

And yes, I do actually evaluate make-disk-image.nix on Darwin ;)

nixos/lib/make-disk-image.nix

@@ -45,19 +45,7 @@ let
     raw   = "img";
   };
 
-  # Copied from https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/installer/cd-dvd/channel.nix
-  # TODO: factor out more cleanly
-
-  # Do not include these things:
-  #   - The '.git' directory
-  #   - Result symlinks from nix-build ('result', 'result-2', 'result-bin', ...)
-  #   - VIM/Emacs swap/backup files ('.swp', '.swo', '.foo.swp', 'foo~', ...)
-  filterFn = path: type: let basename = baseNameOf (toString path); in
-    if type == "directory" then basename != ".git"
-    else if type == "symlink" then builtins.match "^result(|-.*)$" basename == null
-    else builtins.match "^((|\..*)\.sw[a-z]|.*~)$" basename == null;
-
-  nixpkgs = builtins.filterSource filterFn pkgs.path;
+  nixpkgs = lib.cleanSource pkgs.path;
 
   channelSources = pkgs.runCommand "nixos-${config.system.nixosVersion}" {} ''
     mkdir -p $out