Make the pull request base branch and number available in the CI flow

### Required Terms

- [X] I agree to follow this project's [Code of Conduct](https://github.com/heroku/roadmap/blob/main/CODE_OF_CONDUCT.md)
- [X] I have read and accept the [Salesforce Program Agreement](https://www.salesforce.com/company/program-agreement/)

### What service(s) is this request for?

Heroku CI

### Tell us about what you're trying to solve. What challenges are you facing?

We would like to use SonarCloud to analyze the code quality of our project. The Sonar scanner must be executed once the tests have been executed since it uses the coverage result files.

The Sonar scanner needs these parameters when a pull request code must be analyzed: sonar.pullrequest.base, sonar.pullrequest.branch, sonar.pullrequest.key.

The pull request base branch and the pull request key (number) are not available in the CI flow. The branch name is available through the HEROKU_TEST_RUN_BRANCH variable.
It would be really helpful to expose this information in the CI environment variables since it's required to use a tool like Sonarcloud which is a widely used tool in software development.

The Heroku support told us that the pull request number is available in the review app env through the HEROKU_PR_NUMBER variable, but we need it in the CI flow since we need the test coverage files to execute the analysis.