fix(ci): pin ubuntu-20 to run github actions

Author: gpouilloux

.github/workflows/elixir.yml

@@ -10,7 +10,7 @@ jobs:
   build:
 
     name: Build and test
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v2