temporarily set GHA runner to ubuntu 20.04 actions/setup-python#544

Author: tfranzel

.github/workflows/ci.yml

@@ -4,7 +4,7 @@ on: [ push, pull_request ]
 
 jobs:
   prep-tests:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     outputs:
       matrix: ${{ steps.set-vars.outputs.matrix }}
       date: ${{ steps.set-vars.outputs.date }}