v12.0.0-beta.35

Fixed

• A checkpoint that is already running is not handed out a second time.
  Asking for the next step while it runs names the running checkpoint and
  says the build waits for its result.
• Checks and test steps find the tools your own terminal finds, such as
  cargo in ~/.cargo/bin. They used to miss them when the agent's shell
  had a shorter PATH, and a checkpoint failed on "command not found".