action.yml: force use external syntax for uses

This forces the use of the syntax for `uses` calling a specified subfolder of a public repository at a given ref. This is an attempt at moving away from the use of `./` as a path, which forces GitHub Actions to locate the `util/post/action.yml` of the *working directory* and not of the action path. This exists since `uses` cannot take expressions; this would be an easy fix if `./` can be replaced with `${{ github.action_path }}/`. Alas, this is not possible, and this hacky workaround exists instead.

action.yml

@@ -23,7 +23,7 @@ runs:
   using: 'composite'
   steps:
     - name: zerotier
-      uses: ./util/post
+      uses: zerotier/github-action/util/post@main
       with:
         main: |
           set -euo pipefail