CI: use setup-beam (#652)

Author: axelson

.github/workflows/ci.yml

@@ -45,7 +45,7 @@ jobs:
       MIX_ENV: test
     steps:
       - uses: actions/checkout@v2
-      - uses: erlef/setup-elixir@v1
+      - uses: erlef/setup-beam@v1
         with:
           otp-version: ${{matrix.otp}}
           elixir-version: ${{matrix.elixir}}