SERVER API: hardcode platform version to ubuntu-20.04 to run actions-setup-python (pr_checks)

Author: nassauwinter

.github/workflows/pr_checks.yaml

@@ -8,7 +8,7 @@ on:
 jobs:
   Run_PR_checks:
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         python-version: [ '3.6', '3.7', '3.8', '3.9', '3.10' ]