[mergebot] dont run land checks when trunk label is there (#82338)
### Description
<!-- What did you change and why was it needed? -->
As we're adding accept2run, we don't want to do the typical land validation if ciflow/trunk is already on the PR. In this case, we default to the on_green checks to make sure all the PR checks and trunk checks are green before merging it in.
### Issue
https://github.com/pytorch/test-infra/issues/448
### Testing
Tested it locally and made sure land checks was false and on_ green was true
```
LANDCHECKS False ONGREEN True
Attempting merge of https://github.com/pytorch/pytorch/pull/82338 (0.003352566560109456 minutes elapsed)
Traceback (most recent call last):
File "/Users/kerryz/pytorch/.github/scripts/trymerge.py", line 1247, in main
merge(args.pr_num, repo,
File "/Users/kerryz/pytorch/.github/scripts/trymerge.py", line 1178, in merge
find_matching_merge_rule(pr, repo)
File "/Users/kerryz/pytorch/.github/scripts/trymerge.py", line 979, in find_matching_merge_rule
raise RuntimeError(reject_reason)
RuntimeError: Matched rule OSS CI, but PR #82338 has not been reviewed yet
(pytorch) kerryz@kerryz-mbp pytorch %
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82338
Approved by: https://github.com/clee2000, https://github.com/huydhn
