CI: Support automatic publish to crates.io on tag creation

[NobodyXu]

This would make it easier to release new crate, without having to ping anyone.

Whoever has the right to create a new release on GitHub shall have the right to publish it to crates.io

NOTE that this new workflow requires a secret CARGO_REGISTRY_TOKEN to be set. With scoped token, you can limit the token to only have the ability to publish new version under name jobserver, and nothing else.

[NobodyXu]

cc @weihanglo

[NobodyXu]

cc @weihanglo Do you think it's something worth's having for jobserver-rs?

cc-rs is already using this so that I don't have to ping someone on zulip to do the release every time 😂

[weihanglo]

Yeah, and we may want to standardize the release process to include changelog.

Yet I think it's up to @petrochenkov who helps the release process a lot :)