pwsafe: substitute paths in .desktop file

Upstream uses an absolute path in the .desktop file to point to the
icon.

pkgs/applications/misc/pwsafe/default.nix

@@ -39,7 +39,7 @@ stdenv.mkDerivation rec {
     done
 
     # Fix hard coded paths.
-    for f in $(grep -Rl /usr/share/ src) ; do
+    for f in $(grep -Rl /usr/share/ src install/desktop) ; do
       substituteInPlace $f --replace /usr/share/ $out/share/
     done