Update the v2 status banner and pin spawned environments to the running SDK version (#2834)

.github/workflows/shared.yml

@@ -30,9 +30,10 @@ jobs:
         env:
           SKIP: no-commit-to-branch,readme-v1-frozen
 
-      # TODO(Max): Drop this in v2.
+      # TODO(Max): Drop this in v2. Deliberate updates (e.g. the v2 status
+      # banner) go through the 'override-readme-freeze' label.
       - name: Check README.md is not modified
-        if: github.event_name == 'pull_request'
+        if: github.event_name == 'pull_request' && !contains(github.event.pull_request.labels.*.name, 'override-readme-freeze')
         run: |
           git fetch --no-tags --depth=1 origin "$BASE_SHA"
           if git diff --name-only "$BASE_SHA" -- README.md | grep -q .; then