Update ci-build.yml

Author: seratch

.github/workflows/ci-build.yml

@@ -8,7 +8,8 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    # Avoiding -latest due to https://github.com/actions/setup-python/issues/162
+    runs-on: ubuntu-20.04
     timeout-minutes: 10
     strategy:
       matrix: