chore(ci): add pull_request_target workflow for v2-release branch in pr build (#35216)

### Reason for this change

GitHub Actions workflows with `on: pull_request` triggers do not execute
when pull requests are created programmatically via API calls using
personal access tokens. Release process creates PRs using the GitHub
API, causing CI workflows to be skipped and preventing proper validation
of release changes.

### Checklist
- [X ] My code adheres to the [CONTRIBUTING
GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and
[DESIGN
GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache-2.0 license*

Author: ozelalisen

.github/workflows/codebuild-pr-build.yml

@@ -8,6 +8,13 @@ on:
       - main
       - v2-release
   pull_request: {}
+  pull_request_target:
+    branches:
+      - v2-release
+    types:
+      - opened
+      - synchronize
+      - reopened
 
 # For every PR, cancel any previous builds in progress
 # ... but for all other builds we keep them running