argyllcms: unset AR

Commit 093cc00cdd9d8cf31ecce5bc1dd3645c460a1b98 sets the AR environment variable
by default, but this causes the argyllcms Makefile to use the wrong command.

pkgs/tools/graphics/argyllcms/default.nix

···
     substituteInPlace Makefile --replace "-j 3" "-j $NIX_BUILD_CORES"
     # Remove tiff, jpg and png to be sure the nixpkgs-provided ones are used
     rm -rf tiff jpg png
+
+    unset AR
   '';
 
   buildInputs = [