Build a Weather Station with Elixir and Nerves: 'mix deps.get' does not work if user has not created a keyfile (page 16)

Running mix deps.get in the sensor_hub directory fails with the following error:

** (Mix) No SSH public keys found in ~/.ssh. An ssh authorized key is needed to
log into the Nerves device and update firmware on it using ssh.
See your project's config.exs for this error message.

I expect this is because config/target.exs is looking for a public keyfile (specifically, one of three names), but the text doesn’t talk about needing one or how to create one.

OS: macOS 11.5.2
Fresh checkout of asdf v0.8.0
Fresh installs of erlang 24.0.3 and elixir 1.12.2-otp-24.