checkpoints: write dvc.lock on every checkpoint

[casperdcl]

(Over)writing dvc.lock after each checkpoint (so interrupting exp run will mean a usable dvc.lock is present).

Part of iterative/cml#560

[pmrowla]

Just to be clear here, dvc.lock should already be written (updated) at each checkpoint. We should also already be automatically tracking it in git for checkpoints as well. If you're seeing exp/checkpoint commits without dvc.lock files, it sounds like this is a bug somewhere in DVC

[casperdcl]

There may be a bug here but it's low priority if #6182 is solved, so closing for now.