chore(deps): update build-emacs-for-macos to 0.6.40

This should fix nightly build issues caused by homebrew package upgrade
failures.

Author: jimeh

.github/workflows/_prepare.yml

@@ -7,7 +7,7 @@ on:
         description: Git ref to checkout of build-emacs-for-macos
         required: false
         type: string
-        default: "v0.6.39"
+        default: "v0.6.40"
     secrets:
       TAP_REPO_TOKEN:
         description: Personal Access Token for Homebrew Tap repo