gtk: fix missing immodules.cache

this file used to be generated by the gtk build process, but this
stopped being the case when upstream switched to a different
post-install system with version 3.24.32. this is not an upstream bug
since distributions are supposed to generate this file on their own
anyways [1].

Fixes #172406.

[1]: https://gitlab.gnome.org/GNOME/gtk/-/issues/4908

pkgs/development/libraries/gtk/3.x.nix

@@ -201,6 +201,8 @@ stdenv.mkDerivation rec {
     for f in $dev/bin/gtk-encode-symbolic-svg; do
       wrapProgram $f --prefix XDG_DATA_DIRS : "${shared-mime-info}/share"
     done
+  '' + lib.optionalString (stdenv.buildPlatform == stdenv.hostPlatform) ''
+    GTK_PATH="''${out:?}/lib/gtk-3.0/3.0.0/immodules/" ''${dev:?}/bin/gtk-query-immodules-3.0 > "''${out:?}/lib/gtk-3.0/3.0.0/immodules.cache"
   '';
 
   # Wrap demos