Switch CI from ubuntu-latest to ubuntu-20.04 to avoid problems with ansible-test from ansible-core 2.12, 2.13, 2.14. (#138)

(cherry picked from commit 338a8f2cd6)

.github/workflows/ansible-test.yml

@@ -28,7 +28,7 @@ jobs:
           - stable-2.10
           - stable-2.11
           - stable-2.12
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
 
       # ansible-test requires the collection to be in a directory in the form
@@ -78,7 +78,7 @@ jobs:
 # https://docs.ansible.com/ansible/latest/dev_guide/testing_units.html
 
   units:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     name: Units (Ⓐ${{ matrix.ansible }})
     strategy:
       # As soon as the first unit test fails, cancel the others to free up the CI queue
@@ -142,7 +142,7 @@ jobs:
 # https://github.com/ansible-collections/community.zabbix/tree/master/.github/workflows
 
 #  integration:
-#    runs-on: ubuntu-latest
+#    runs-on: ubuntu-20.04
 #    name: I (Ⓐ${{ matrix.ansible }}+py${{ matrix.python }}})
 #    strategy:
 #      fail-fast: false