Don't start land checks if the PR hasn't been approved yet (#84239)
Per title, don't start land checks if the PR hasn't been approved yet. This is very important to make sure that we don't start CI jobs from unknown devs, i.e. first time contributor.
Also rename force to `skip_mandatory_checks` to make it clearer on what this flag does
### Testing
```
python .github/scripts/test_trymerge.py
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84239
Approved by: https://github.com/zengk95, https://github.com/ZainRizvi
