Stick to the ubuntu-20.04 (#2472)

the ubuntu-latest is switching to 22.04 and discontinued support of python 2.7 and 3.6

.github/workflows/tests.yaml

@@ -10,7 +10,7 @@ on:
 
 jobs:
   unit:
-    runs-on: ${{ matrix.os }}-latest
+    runs-on: ${{ fromJson('{"ubuntu":"ubuntu-20.04","windows":"windows-latest","macos":"macos-latest"}')[matrix.os] }}
     strategy:
       fail-fast: false
       matrix:
@@ -89,7 +89,7 @@ jobs:
       run: python -m coveralls --service=github
 
   behave:
-    runs-on: ${{ matrix.os }}-latest
+    runs-on: ${{ fromJson('{"ubuntu":"ubuntu-20.04","windows":"windows-latest","macos":"macos-latest"}')[matrix.os] }}
     env:
       DCS: ${{ matrix.dcs }}
       ETCDVERSION: 3.3.13