[infra] authenticate version-updater via flutteractionsbot

[pq]

Our automated Flutter SDK version-bump presubmit was failing to trigger subsequent CI/CD test workflows and was getting blocked by Google's CLA check. (See for example: #8955.)

• Standard GITHUB_TOKEN creations do not trigger downstream actions on PR creation (preventing presubmit tests from running on the PR).
• system github-actions[bot] commits are not registered with Google's CLA bot, blocking the merge path.

This PR configures the workflow to authenticate using our organization's pre-approved flutteractionsbot credentials, enabling instant CLA compliance and automatic presubmit test execution. (Note I had to create a fork of this repo for flutteractionsbot behind the scenes, explaining some action failures leading up to the success below).

I verified this works by manually kicking off the action, which produced this PR:

#8958

which has the right CLA credentials:

Completes the work started in #8953.

🚀 🚀 🚀

---

Proposed Changes

• [Modified] .github/workflows/update_flutter.yaml:
  • Configured the Checkout and Create Pull Request steps to authenticate via the organization's secrets.FLUTTERACTIONSBOT_CP_TOKEN PAT.
  • Explicitly overrode committer and author git metadata to match flutteractionsbot <flutter-actions-bot@google.com> to pass Google's CLA checks automatically.
  • Configured push-to-fork: flutteractionsbot/flutter-intellij to push the PR branch to the bot's fork, bypassing upstream main-branch push restrictions.

---

Verification Results

• JUnit Integrity Tests: Checked locally using ./gradlew test --tests io.flutter.CIIntegrityTest — BUILD SUCCESSFUL (all meta-tests compile and pass).
• YAML Syntax Vetted: Validated structured parameters against the latest peter-evans/create-pull-request specifications.

Replaces #8956

---

Review the contribution guidelines below:

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.
• I've included the required information in the description above.
• My up-to-date information is in the AUTHORS file.
• I've updated CHANGELOG.md if appropriate.

Contribution guidelines:

• See
  our contributor guide and
  the Flutter organization contributor guide
  for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Dart contributions to our repos should follow the Dart style guide and use
  dart format.
• Java and Kotlin contributions should strive to follow Java and Kotlin best
  practices (discussion).

[gemini-code-assist]

Note

Gemini is unable to generate a review for this pull request due to the file types involved not being currently supported.