Produce nightly.yaml release YAML with stable name

[imjasonh]

We should produce a nightly release YAML with a stable name, so users don't have to do any work or write a script to be able to install the latest nightly.

GitHub Releases don't let us have two release assets with the same name, so we'll have to update the nightly release workflow to delete any existing nightly{-debug}.yaml before adding a new one for the latest nightly YAML.

We should continue to produce date-versioned nightly release YAMLs, which can be useful for debugging issues.