Change occurences of actions/setup-elixir to erlef/setup-elixir (#2)

* Change all occurrences of actions/setup-elixir to erlef/setup-elixir

.github/workflows/test.yml

@@ -39,7 +39,7 @@ jobs:
             elixir-version: '1.11.0-rc.0'
     steps:
       - uses: actions/checkout@v2
-      - name: Use actions/setup-elixir
+      - name: Use erlef/setup-elixir
         id: setup-elixir
         uses: ./
         with: