Continue using older Ubuntu for older Elixir releases

Author: michalmuskala

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ on:
 jobs:
   mix_test:
     name: mix test (Elixir ${{matrix.elixir}} | OTP ${{matrix.otp}})
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     strategy:
       fail-fast: false
       matrix: