tlaplus-toolbox: fix crash when opening file dialog

This is re-applying the fix from
https://github.com/NixOS/nixpkgs/pull/129042, which I think was
accidentally partially reverted in
https://github.com/NixOS/nixpkgs/pull/441183.

In particular, the application can't find gsettings schemas to open the
file dialog and prints this error message:

    No GSettings schemas are installed on the system

The search paths for these schemas is normally set up by
gappsWrapperArgsHook in a preFixupPhase, but this package creates the
wrapper in the installPhase, so gappsWrapperArgs isn't fully initialized
yet. I believe this hook normally runs after install so the glib
postInstall hook can also detect schemas in $out, but this package
doesn't have any.

It's sufficient to just call gappsWrapperArgsHook and not call
fixupPhase like in the original PR. gappsWrapperArgsHook is one of the
preFixupPhases, not part of preFixup, so calling fixupPhase won't run
it.

pkgs/by-name/tl/tlaplus-toolbox/package.nix

@@ -60,6 +60,9 @@ stdenv.mkDerivation (finalAttrs: {
     patchelf --set-interpreter ${bintools.dynamicLinker} \
       "$(find "$out/libexec/toolbox" -name jspawnhelper)"
 
+    # Populate gappsWrapperArgs now instead of in a preFixupPhase.
+    gappsWrapperArgsHook
+
     makeShellWrapper $out/libexec/toolbox/toolbox $out/bin/tla-toolbox \
       --chdir "$out/libexec/toolbox" \
       --add-flags "-data ~/.tla-toolbox" \