chore: update readme for scorecard (#456)

### Description

Small update README to reflect that the job can also be triggered by
adding the execute_gs_scorecard label to a pull request.

### Checklist

- [ ] `README.md` has been updated or is not required
- [ ] push trigger tests
- [ ] manual release test
- [ ] automated releases test
- [ ] pull request trigger tests
- [ ] schedule trigger tests
- [ ] workflow errors/warnings reviewed and addressed

### Testing done 
(for each selected checkbox, the corresponding test results link should
be listed here)

Author: mbruzda-splunk

README.md

@@ -532,7 +532,7 @@ appinspect-api-html-report-self-service
 
 - The GS Scorecard tool is containerized and runs in a Docker container, analyzing the repository and generating a comprehensive quality report.
 
-- This job only runs on push events to the `main` branch after a successful build.
+- This job runs only after a successful build, either on push events to the main branch or when the execute_gs_scorecard label is added to a pull request.
 
 **Action used:** 
 - AWS ECR (Elastic Container Registry) for Docker image storage