emacsPackages.ws-butler: avoid `fetchFromSavannah` (#477666)

          workgroups2 = ignoreCompilationError super.workgroups2; # elisp error

          ws-butler = super.ws-butler.overrideAttrs (old: {

            # work around https://github.com/NixOS/nixpkgs/issues/436534

            src = pkgs.fetchFromSavannah {

              repo = "emacs/nongnu";

              inherit (old.src) rev outputHash outputHashAlgo;

            # TODO: Remove override when URL was updated in MELPA.

            src = old.src.override {

              url = "https://https.git.savannah.gnu.org/git/elpa/nongnu.git";

            };

          });