[land-bot] Only operate on trusted patches

Summary:
Even though the land bot will be running in a jail, so the results of the
build do not matter even if they are horribly clobbered, it's possible for the build
to leak sensitive information such as the SSH private key that will be used to push
changes upstream. To ensure this is not possible, all revisions going through land
bot will at least need to be already greened by folks in the Bitcoin ABC reviewer group.

Test Plan:

CONDUIT_TOKEN=<your-token> ./land-patch.sh --dry-run -r Dxxxx

Where xxxx point to revisions with different statuses:

• Accepted by Bitcoin ABC reviewers (patch is applied, smoke tests run, etc.)
• Rejected by Bitcoin ABC reviewers (fails as expected)
• Not reviewed by Bitcoin ABC reviewers (fails as expected)

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6017