common-updater-scripts: Add file and system flags

You can now optionally invoke update-source-versions with:

* --system flag changing the host platform, to be passed dirrectly to Nix commands.
  This is useful for binary packages which have different sources for each platform.
* --file flag allowing to change the file to be modified. This is useful for packages
  that offer multiple variants, listed in a different file than the derivation itself;
  e.g. packages.nix of Sublime Text 3.
* --version-key, which is now a keyword flag instead of a positional argument.

pkgs/common-updater/scripts.nix

@@ -1,4 +1,4 @@
-{ stdenv, makeWrapper, coreutils, gawk, gnused, diffutils, nix }:
+{ stdenv, makeWrapper, coreutils, gawk, gnused, gnugrep, diffutils, nix }:
 
 stdenv.mkDerivation {
   name = "common-updater-scripts";
@@ -12,7 +12,7 @@ stdenv.mkDerivation {
     cp ${./scripts}/* $out/bin
 
     for f in $out/bin/*; do
-      wrapProgram $f --prefix PATH : ${stdenv.lib.makeBinPath [ coreutils gawk gnused nix diffutils ]}
+      wrapProgram $f --prefix PATH : ${stdenv.lib.makeBinPath [ coreutils gawk gnused gnugrep nix diffutils ]}
     done
   '';
 }