Failing with latest emacs due to withGTK3 defaulting to false

[epetousis]

Related to nix-community/emacs-overlay#244 I believe, looks like the latest nixpkgs-unstable emacs derivation sets withGTK3 to false by default, but withXwidgets requires it to be true.

The error I'm getting when trying to build:

error: assertion '(withXwidgets -> (withGTK3 && (webkitgtk != null)))' failed

       at /nix/store/1hyrj236ljnvh1q3f7039877vd4sgrq9-source/pkgs/applications/editors/emacs/generic.nix:52:1:

           51| assert withPgtk -> withGTK3 && !withX && gtk3 != null;
           52| assert withXwidgets -> withGTK3 && webkitgtk != null;
             | ^
           53|