Fix issue bot triggering on PRs (#38525)

Summary:
This PR fixes a bug with an issue labeling bot issues triggering on comments to pull requests. The action doesn't trigger when a pull request is open (but other actions add comments immediately).

From [GitHub docs](https://docs.github.com/en/webhooks-and-events/events/issue-event-types):

> GitHub's REST API considers every pull request to be an issue, but not every issue is a pull request. [...] Pull requests have a **`pull_request` property in the `issue` object.**

☝️ Which is a way to differentiate between an issue and a pull request

Relates to a bug introduced in #38338 and links to ☂️#35591

## Changelog:

[INTERNAL] [FIXED] - Prevent issue bot triggering on pull requests

Pull Request resolved: #38525

Test Plan:
This PR makes use of  `github.event.issue.pull_request` object which is null on issues and comments in issues:

![image](https://github.com/facebook/react-native/assets/39658211/e0a64039-5fb7-4ad3-95aa-65cb7c9f0a4b)

and truthy on comments on pull requests

![image](https://github.com/facebook/react-native/assets/39658211/9df69d00-c792-4c00-bfc2-9ee832551d16)

With a change from PR bot skips execution on comments on pull requests

![image](https://github.com/facebook/react-native/assets/39658211/f02687fb-f81b-4278-8d0e-9b23651229c1)

Reviewed By: ryancat

Differential Revision: D47628330

Pulled By: cortinico

fbshipit-source-id: 63e7180bb57ffd34904414a702c02019e18a042b

.github/workflows/check-for-reproducer.yml

@@ -10,7 +10,7 @@ on:
 jobs:
  check-for-reproducer:
  runs-on: ubuntu-latest
- if: github.repository == 'facebook/react-native'
+ if: github.repository == 'facebook/react-native' && github.event.issue.pull_request == null
  steps:
  - uses: actions/checkout@v3
  - uses: actions/github-script@v6