libstrangle: use direct nix store path in layer json

This allows you to load libstrangle without setting LD_LIBRARY_PATH to include
it. Only ENABLE_VK_LAYER_TORKEL104_libstrangle=1 is required now, as expected of
an implicit layer.

Previously, you were required to run your VK app via the wrapper:

STRANGLE_FPS=30 strangle vkcube

Now you can control it via simple environment variables alone:

ENABLE_VK_LAYER_TORKEL104_libstrangle=1 STRANGLE_FPS=30 vkcube

pkgs/tools/X11/libstrangle/default.nix

···
     substituteAllInPlace src/strangle.sh
     substituteAllInPlace src/stranglevk.sh
   '';
+  postInstall = ''
+    substituteInPlace $out/share/vulkan/implicit_layer.d/libstrangle_vk.json \
+      --replace "libstrangle_vk.so" "$out/lib/libstrangle/lib64/libstrangle_vk.so"
+  '';
 
   meta = with lib; {
     homepage = "https://gitlab.com/torkel104/libstrangle";