update runner type

.github/workflows/lint.yaml

@@ -8,7 +8,7 @@ concurrency:
 
 jobs:
   lint:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v2
       - name: Set up Python 3.6

.github/workflows/publish.yaml

@@ -8,7 +8,7 @@ concurrency:
 
 jobs:
   build-n-publish:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     if: startsWith(github.event.ref, 'refs/tags')
     steps:
       - uses: actions/checkout@v2