gnuradio3_{7,8}: Use external volk

Now that volk is packaged, we can use our build and save some closure
space for those that have more then 1 version of gnuradio installed.
Also, simplify a bit how attributes are inherited in the expression.

pkgs/development/gnuradio-modules/mkDerivation.nix

@@ -17,7 +17,9 @@ let
   args_ = {
     enableParallelBuilding = args.enableParallelBuilding or true;
     nativeBuildInputs = (args.nativeBuildInputs or []);
-    # We add gnuradio itself by default
-    buildInputs = (args.buildInputs or []) ++ [ unwrapped ];
+    # We add gnuradio and volk itself by default - most gnuradio based packages
+    # will not consider it a depenency worth mentioning and it will almost
+    # always be needed
+    buildInputs = (args.buildInputs or []) ++ [ unwrapped unwrapped.volk ];
   };
 in mkDerivation (args // args_)