buildbot_nix: update comment

This doesn't invoke nix-instantiate anymore, but
`nix-eval-jobs --flake .#checks`.

buildbot_nix/__init__.py

@@ -131,7 +131,7 @@ def __init__(self, supported_systems: list[str], **kwargs: Any) -> None:
 
  @defer.inlineCallbacks
  def run(self) -> Generator[Any, object, Any]:
- # run nix-instanstiate to generate the dict of stages
+ # run nix-eval-jobs --flake .#checks to generate the dict of stages
  cmd: remotecommand.RemoteCommand = yield self.makeRemoteShellCommand()
  yield self.runCommand(cmd)