chore(ci): extend and rename nightly workflow to support RC builds

[vpavlin]

Description

This PR updates Nightly build workflow to also support building and create release candidate (RC) tags and releases.

It also adds version (e.g. nightly, v0.18.0-rc.0...) to artifact filenames.

I also removed the cache as it might make sense to "build from scratch" for releases, but I don't have a strong opinion on this

You can see the example pre-releases here: https://github.com/nwaku-test-org/nwaku/releases

I've also RC release steps to the release process docs

FYI We have a v0.18.0 release scheduled for next week, so I'd like to use this workflow on Friday to release the v0.18.0-rc.0

Changes

• rename nightly to pre-release workflow
• add support for *-rc.X releases to the workflo
• add info about RC releases to release process doc

[jm-clius]

LGTM. Thanks for improving this process.

[vpavlin]

@rymnc the experimental test seems to be a bit flaky here:)

[rymnc]

Let me check and get back to you @vpavlin

[Ivansete-status]

This looks awesome! Thanks for it!
I think it is a good idea to generate RC from scratch.
On the other hand, I wouldn't do anything around wakunode1 (build & package) as we are going to deprecate it someday soon.

[Ivansete-status]

I think we are almost deprecating the wakunode1.
I wouldn't add it to the release package, just to avoid confusion.

[vpavlin]

It is currently part of it (https://github.com/waku-org/nwaku/blob/master/.github/workflows/release-assets.yml#L57), so I wanted to make sure we keep things relatively unchanged until the repo refactoring is done

[Ivansete-status]

Ok cool, thanks for letting me know!

[vpavlin]

@rymnc seems like the test passed this time (3rd run) - would be probably good to check if there is a possibility of some kind of race condition (or what other "flaky" CI behaviour could cause this - e.g. if resource limited environment can influence the outcome of the test