Fix failing Elixir setup step on CI

.github/workflows/ci.yml

@@ -9,12 +9,12 @@ on:
 jobs:
   format:
     name: Check Formatting
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-16.04
     steps:
       - uses: actions/checkout@v2
 
       - name: Set up Elixir
-        uses: actions/setup-elixir@v1
+        uses: erlef/setup-elixir@v1
         with:
           otp-version: 22.2
           elixir-version: 1.10.4
@@ -27,7 +27,7 @@ jobs:
 
   test:
     name: Build and test
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-16.04
     strategy:
       fail-fast: false
       matrix:
@@ -48,7 +48,7 @@ jobs:
       - uses: actions/checkout@v2
 
       - name: Set up Elixir
-        uses: actions/setup-elixir@v1
+        uses: erlef/setup-elixir@v1
         with:
           otp-version: ${{matrix.pair.erlang}}
           elixir-version: ${{matrix.pair.elixir}}