fleshed out "nightly builds" system

A couple releases ago we shipped an undocumented feature called release-branch:

https://github.com/axodotdev/cargo-dist/blob/d97fc68880d963781aec462c72cd2c58cd3d142c/cargo-dist/src/config.rs#L245-L247

This feature is for a gitops-style "every push to this branch is a deploy". As part of this we force cargo-dist to select all publishable apps, take the maximum of their versions, and then append `-alpha.{timestamp}` to that version, and then pretend the all apps have that version:

https://github.com/axodotdev/cargo-dist/blob/d97fc68880d963781aec462c72cd2c58cd3d142c/cargo-dist/src/announce.rs#L511-L525

A lot of this is the machinery you might want for a "nightly release" but there are some potential differences/improvement:

* The cargo packages aren't actually modified to have the given version (so clap-based --version flags won't mention it's a nightly)
* While in a concrete case this is straight-forward, in the general case things are quite complicated with things like workspace-version-inheritance. I briefly looked into vendoring some code from release-plz but ended up discarding it when i got to the end.
* If you care about commiting and tagging your nightlies this raises a lot of questions about modifying the tree permanently
* For a "nightly" workflow you might want something more akin to a cron job than "every push"
* As seen in the comments, it would be nice for the version string to be a template

Start with the release-branch configuration in cargo-dist/src/config.rs and the version-selection logic in cargo-dist/src/announce.rs. Compare the existing behavior with the listed nightly-build needs; the scope is not complete until version handling, trigger style, and version-template expectations are settled.

Written by the indexing model from the issue text.