Github Actions: use ubuntu-20.04 instead of ubuntu-latest

ubuntu-latest is changing to ubuntu-22.04 with no Python 2.7 support
anymore.

actions/runner-images#6399

Author: thiell

.github/workflows/nosetests.yml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         python-version: [ '2.7', '3.6', '3.7', '3.8', '3.9' ]