apmplanner2: fixed wrong path in .desktop file

The `apmplanner2.desktop` file defaults to `/usr/share/...` regardless which out
prefix is given to make. So I chose to fix it manually.

pkgs/applications/science/robotics/apmplanner2/default.nix

@@ -23,8 +23,12 @@ stdenv.mkDerivation rec {
   qmakeFlags = [ "apm_planner.pro" ];
 
   # this ugly hack is necessary, as `bin/apmplanner2` needs the contents of `share/APMPlanner2` inside of `bin/`
-  preFixup = "ln --relative --symbolic $out/share/APMPlanner2/* $out/bin/";
-
+  preFixup = ''
+    ln --relative --symbolic $out/share/APMPlanner2/* $out/bin/
+    substituteInPlace $out/share/applications/apmplanner2.desktop \
+                      --replace /usr $out
+  '';
+  
   enableParallelBuilding = true;
 
   meta = {