[land-bot] Skip sanity check during arc patch

Summary:
The sanity check step of arc patch is primarily in place to
prevent accidental landing of a revision before it's base commit.
This check is problematic when the landbot has queued multiple dependent
revisions, as landing the first revision will alter its commit hash and
fail subsequent sanity checks.

This patch uses --force to skip those sanity checks. This is generally
safe since we run our own smoke tests prior to landing, so this will catch
any master-breaking changes that do not encounter merge conflicts.

Test Plan:
Find an unlanded revision that depends on another and has not been rebased on master.
./autopatch -r <that-revision>
Should succeed post-patch.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6691