chore(ci): scope codebuild PR build to aws/aws-cdk (#35638)

### Description of changes

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Forks do not have any codebuild setup, so this pr-build should only run
for aws/aws-cdk. Currently, it creates noise and the action stays in
pending, eventually timing out.
https://github.com/aws/aws-cdk/actions/workflows/pr-build.yml action
runs on forks to test PR Build

### 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

@@ -24,6 +24,8 @@ concurrency:
 
 jobs:
   build:
+    # Runner exists only for aws-cdk repo
+    if: github.repository == 'aws/aws-cdk'
     runs-on: codebuild-aws-cdk-github-actions-runner-${{ github.run_id }}-${{ github.run_attempt }}
 
     env: