Change runner ubuntu version to fix GitHub error

See also:
https://github.com/actions/setup-python/issues/162#issuecomment-1325307787

.github/workflows/continous-integration.yml

@@ -12,7 +12,7 @@ on:
 jobs:
   test_node:
     name: Node CI
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     env:
       ALLOWED_HOSTS: localhost,mozfest.localhost,default-site.com,secondary-site.com
       GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
@@ -32,7 +32,7 @@ jobs:
 
   test_wagtail:
     name: Wagtail CI
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     services:
       postgres:
         image: postgres:13.2
@@ -103,7 +103,7 @@ jobs:
 
   test_percy:
     name: Percy CI
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     services:
       postgres:
         image: postgres:13.2
@@ -168,7 +168,7 @@ jobs:
 
   test_integration:
     name: Integration testing
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     services:
       postgres:
         image: postgres:13.2