nixos: make-system-tarball, add option for extra arguments for tar

Sometimes extra arguments when making tarball are required,
for example if making a container owner of files has to be changed
to root.

nixos/lib/make-system-tarball.sh

@@ -50,7 +50,7 @@ done
 
 mkdir -p $out/tarball
 
-tar cvJf $out/tarball/$fileName.tar.xz *
+tar cvJf $out/tarball/$fileName.tar.xz * $extraArgs
 
 mkdir -p $out/nix-support
 echo $system > $out/nix-support/system