fix issues with Cargo.lock

[mimoo]

imagine the flow if we add/remove/update a dependency:

1. cargo build the PR's last commit
2. cargo detects that some dependency was changed, so it updates Cargo.lock
3. this step checkouts the base, which will trigger a warning and fail:

error: Your local changes to the following files would be overwritten by checkout:
	Cargo.lock
Please commit your changes or stash them before you switch branches.
Aborting