feat(builds): improve build speeds and use free arm64 runners

Improves a few things:

- Use the free macos-14 runners for arm64 builds. They are not as fast
  the macos-13-xlarge runners used before, but they are free for public
  repositories, meaning we can start doing nightly arm64 builds.
- Use different nix install and cache actions which are faster, and uses
  a single action cache key, which avoids cache rate limit errors
  which was slowing down cache create/restore times.
- Generally refactor and tidy up various bits and pieces in workflows.

.github/workflows/_update-casks.yml

@@ -11,10 +11,6 @@ on:
         type: string
         required: false
         default: "ubuntu-latest"
-    secrets:
-      TAP_REPO_TOKEN:
-        description: Personal Access Token for Homebrew Tap repo
-        required: true
 
 jobs:
   emacs-builds: