project.el: Fix bug in project-ignores method for VC-aware backend

The variable `backend' was always nil preventing the 25-line long
`when' block from doing anything.  This bug was introduced in commit
785fa80159 "New user option:
project-vc-extra-root-markers". (Bug#65704)

* lisp/progmodes/project.el (project-ignores): Make sure the variable
is initialized, so that backend-specific code is used if any.

lisp/progmodes/project.el

@@ -733,11 +733,10 @@ See `project-vc-extra-root-markers' for the marker value format.")
 
 (cl-defmethod project-ignores ((project (head vc)) dir)
   (let* ((root (nth 2 project))
-         backend)
+         (backend (cadr project)))
     (append
      (when (and backend
                 (file-equal-p dir root))
-       (setq backend (cadr project))
        (delq
         nil
         (mapcar