ci(prbuild): update pr-build workflow to not run on aws-cdk repo (#34932)

### Reason for this change

Stops pr-builds on aws/aws-cdk repository

### Description of changes
Workflow disabled using an if condition in the workflow yaml


- [ ] 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: kumvprat

.github/workflows/pr-build.yml

@@ -20,6 +20,7 @@ concurrency:
 
 jobs:
   build:
+    if: github.repository != 'aws/aws-cdk'
     runs-on: aws-cdk_ubuntu-latest_32-core
 
     env: