chore(ci): update pr-build job name to avoid conflict (#35121)

### Reason for this change



Both PR Build workflow files have same job name which is confusing mergify 

### 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)

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*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/pr-build.yml

@@ -19,7 +19,7 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
-  build:
+  pr-build:
     if: github.repository != 'aws/aws-cdk'
     runs-on: aws-cdk_ubuntu-latest_32-core