Candidate on CI: implement `latest` or commit slug in the link

I have made a nice release process for Hackage packages using GitLab CI _(example run: https://gitlab.com/migamake/homplexity/pipelines/120317820, source code: https://gitlab.com/migamake/homplexity/-/blob/master/.gitlab-ci.yml)_

I even upload a candidate to make sure it is release ready.
However, I cannot put the link to the current candidate, since CI can only use Git Commit hash or branch name as variable name.
So I need to go into candidate upload job log, like here: https://gitlab.com/migamake/homplexity/-/jobs/447092421
And find the link myself, or try to recall which version of the package is produced:
https://hackage.haskell.org/package/homplexity-0.4.6.0/candidate

Since some CDs allow for URL to be included, it would be fixed by either:
1. Providing https://hackage.haskell.org/package/homplexity-latest/candidate link
2. Or allowing one to put upload hash for the candidate
```
cabal upload homplexity.tar.gz --candidate-hash c90c0148
```
https://hackage.haskell.org/package/homplexity-c90c0148/candidate

There is no necessity to store it longer than few weeks, or when new candidate is uploaded, but it would allow one to double check package description, and documentation before release.

That would be related to #597, and possibly allow to implement #558 better with the help of external CI/CD.