Fix build problems: switch to Ubuntu-20.04.

We need Python 2.7 to build.

Author: aljungberg

.github/workflows/main.yml

@@ -4,7 +4,7 @@ on:
     branches: [ master ]
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: Ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v2