haunt: fix Guile load paths

Guile's version is part of the path and the installed files won't be
found if we don't include it.  Also, we can rely on the paths of build
inputs being added to $GUILE_LOAD_PATH and $GUILE_LOAD_COMPILED_PATH
by Guile's setup hook.

pkgs/applications/misc/haunt/default.nix

@@ -29,11 +29,15 @@ stdenv.mkDerivation rec {
 
   doCheck = true;
 
-  postInstall = ''
-    wrapProgram $out/bin/haunt \
-      --prefix GUILE_LOAD_PATH : "$out/share/guile/site:${guile-commonmark}/share/guile/site:${guile-reader}/share/guile/site" \
-      --prefix GUILE_LOAD_COMPILED_PATH : "$out/share/guile/site:${guile-commonmark}/share/guile/site:${guile-reader}/share/guile/site"
-  '';
+  postInstall =
+    let
+      guileVersion = lib.versions.majorMinor guile.version;
+    in
+    ''
+      wrapProgram $out/bin/haunt \
+        --prefix GUILE_LOAD_PATH : "$out/share/guile/site/${guileVersion}:$GUILE_LOAD_PATH" \
+        --prefix GUILE_LOAD_COMPILED_PATH : "$out/lib/guile/${guileVersion}/site-ccache:$GUILE_LOAD_COMPILED_PATH"
+    '';
 
   doInstallCheck = true;
   installCheckPhase = ''