Move Stack and Nix builds to GitLab CI

[martijnbastiaan]

No description provided.

[christiaanb]

Will this still run the circle ci builds for external contributors?

[martijnbastiaan]

"Normal" tests: yes, stack/nix: no. There's already a number of tests GitLab CI only (haddock, bindist tests), so I figured we should just treat Circle CI as the first line of defense and use its minutes sparingly. If we see any dependency changes in a PR, we should just clone and push to a local directory branch to run everything on GitLab.

I think we should spend a day on building a bot that does the clone+push for us on a, say, @clash-lang-builder run ci comment. I've talked to @leonschoorl and @alex-mckenna (independently, just during "coffee moments" so-to-speak) about this before. It should be pretty easy with GitHub Actions and/or probot:

• https://github.com/probot/example-github-action
• https://github.com/probot/adapter-github-actions
• GitHub Actions's issue_comment: https://docs.github.com/en/actions/reference/events-that-trigger-workflows#issue_comment

Edit: Done

[martijnbastiaan]

Going to go ahead and merge this so we can use it for #1708.