fix: anchored ubuntu version to support Python 2.7

- See: actions/setup-python#543
- Context: https://github.com/nodejs/build/actions/runs/3519497549/jobs/5899537570

.github/workflows/push.yml

@@ -2,7 +2,7 @@ name: Lint
 on: [push, pull_request]
 jobs:
   Python:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       max-parallel: 2
       matrix: