[ci] Use specific ubuntu OS

Author: cpjolicoeur

.github/workflows/v3.yml

@@ -10,7 +10,7 @@ on:
 jobs:
   mix_test:
     name: mix test (Elixir ${{matrix.elixir}} | OTP ${{matrix.otp}})
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     strategy:
       fail-fast: false