molly-guard: Reduce closure size by stripping unneeded dependeicies

pkgs/os-specific/linux/molly-guard/default.nix

···
 
   buildInputs = [ dpkg ];
 
-  sourceRoot = ".";
-
   unpackCmd = ''
-    dpkg-deb -x "$src" .
+    dpkg-deb -x "$src" source
   '';
 
   installPhase = ''