feat(build): allow running build job on separate OS

This should help reduce costs for Apple Silicon builds, as they are not
available on any form of free tier of GitHub Actions at the moment.

The build step that actually runs Emacs' configure script, and compiles
Emacs, is the only step that actually needs to run on the target
hardware architecture. The rest should be architecture agnostic.

.github/workflows/_prepare.yml

@@ -6,7 +6,8 @@ on:
       os:
         description: GitHub Actions runner OS
         type: string
-        required: true
+        required: false
+        default: "macos-11"
       builder_ref:
         description: Git ref to checkout of build-emacs-for-macos
         required: false