Add link to GitHub pull request

[clayburn]

Summary

• When building on GitHub Actions for a pull request, adds a "GitHub pull request" link to the Build Scan
• Extracts the PR number from the GITHUB_REF_NAME environment variable (format <number>/merge)
• Mirrors the equivalent change in the Gradle plugin: Add link to GitHub pull request common-custom-user-data-gradle-plugin#472

Test plan

• Verify the build compiles and existing tests pass
• Trigger a GitHub Actions PR build and confirm the "GitHub pull request" link appears in the Build Scan

[dotta]

Shared my feedback, but I'm leaving it to @Duhemm to accept or request changes.

[dotta]

Why is it required for headRef to be Some? If it's indeed required, I'd lift this out and make it a conditional for the whole expression, i.e.,

if (headRef.nonEmpty) {
  val pullRequestUrl = for { ... 
}

This because I believe it makes the intended logic a lot more apparent and easier to reason about.

[Duhemm]

I think it might make sense to move the PR number extraction logic to a dedicated method so that it's obvious that it'll return something only when we're running in a PR. That'll also address Mirco's concern about the guard being a bit obscure.