CI UI: emit markup for grouping log lines

[maxheld83]

I'm having a hard time navigating flake-iter logs in CI (they're long 😅).

I'll use GitHub Actions as an example to motivate this.

I'd like to group the log lines by derivation for easier navigation.

So this:

2025-03-13T12:14:56.332927Z DEBUG flake_iter::cmd::build: Building derivation 1 of 4 drv="/nix/store/naagvgp1cpkhs8ab653a34qlah8b349d-treefmt-check.drv^*"
this derivation will be built:
  /nix/store/naagvgp1cpkhs8ab653a34qlah8b349d-treefmt-check.drv

Should mark the starting of something like this:

Happy to take a swing at this, but wanted to check in first.

[maxheld83]

Some initial thoughts:

1. This is strictly a GHA issue, but the easiest/only way to implement might still be flake-iter.
   The alternative would be to post-process the logs in https://github.com/determinatesystems/ci, or some intermediary package/(flake?), but would probably necessitate a lot of regex and would introduce tight coupling between how flake-iter logs and some other software (weird -- there aren't usually stability guarantees about log formatting).
2. GitLab has a similar feature, and probably more tools have similar features, though each with different markup see here.
   It there seems like a good idea to make this (the used markups) extensible, and not to hard-code GHA in flake-iter.
3. There should be a (rust) logging library which already deals with this, and just intersperses the appropriate markup at flake-iter build-time, but I couldn't find such a tool.

Questions:

• re: 2) Should this be GHA-only, or is it worth it to make this extensible?
• re: 3) Should this even be in flake-iter? (The more I think about it, the more I think the whole logging-to-CI-markup-thing should be a feature of a logging library ...).

[maxheld83]

uh, I think doing this properly without scope-creeping flake-iter requires a lot more thought, and probably a separate library or finding one which does this already.

Might come back if I have a more constructive idea, closing for now.

Sorry about the noise.