Update elixir.yml

ubuntu-20.04

.github/workflows/elixir.yml

@@ -2,7 +2,7 @@ name: mix
 on: push
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v2
     - uses: erlef/setup-elixir@v1