Accept stacked PR base branches in analyze-pr

[huntie]

Summary

The analyze_pr check fails every PR in a stack above the first (example: #58867), since each targets the branch of the PR below it rather than main or a -stable branch.

• Pass the branch target check when the base branch is the head of another open PR in the repo.
• Other base branches still fail as before, and the parent PR's own check still guards its target.

NOTE: The workflow runs on pull_request_target from main, so this takes effect only after it lands.

Changelog: [Internal]

Test Plan

yarn jest .github/workflow-scripts/__tests__/checkBranchTarget-test.js

Checked the lookup the workflow makes against GitHub:

gh api 'repos/react/react-native/pulls?state=open&head=react:pr58950'

✅ Returns #58950, so #58951 (based on pr58950) will pass.
ℹ️ A non-PR branch returns no results and still fails.