system-syzygy: Fix invalid desktopFile arguments

Fixes this build failure:

Running desktop-file validation
/nix/store/vadj03624kv6h963c7fjwdz1hg403aa9-system-syzygy.desktop/share/applications/system-syzygy.desktop: error: value "%out%/bin/syzygy" for key "Exec" in group "Desktop Entry" contains an invalid field code "%o"
/nix/store/vadj03624kv6h963c7fjwdz1hg403aa9-system-syzygy.desktop/share/applications/system-syzygy.desktop: error: value "%out%/bin/syzygy" for key "Exec" in group "Desktop Entry" contains an invalid field code "%/"

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

pkgs/games/system-syzygy/default.nix

@@ -3,7 +3,7 @@
 let
   desktopFile = makeDesktopItem {
     name = "system-syzygy";
-    exec = "%out%/bin/syzygy";
+    exec = "@out@/bin/syzygy";
     comment = "A puzzle game";
     desktopName = "System Syzygy";
     categories = "Game;";