feat(issue-labeler): support pull_request_target events

[yonib05]

Summary

• Use fallback expressions (github.event.issue.* || github.event.pull_request.*) for ISSUE_TITLE, ISSUE_BODY, and ISSUE_NUMBER env vars
• Allows the action to work for both issues and pull_request_target workflow triggers without changes

Test plan

• Verify action still works for issue events (existing behavior)
• Verify action works for pull_request_target events (new behavior)