GHA UI: expose build derivations x system as job summary

[maxheld83]

I like this output from https://github.com/DeterminateSystems/flake-iter:

2025-03-13T16:57:22.773389Z DEBUG flake_iter::cmd::build: Building derivation 1 of 4 drv="/nix/store/3bp7mzy56dk29qwwzry4qdhm598s2wmq-nix-unit-check.drv^*"
2025-03-13T16:57:24.100699Z DEBUG flake_iter::cmd::build: Building derivation 2 of 4 drv="/nix/store/k3h08g3dwwapa92797y6c959m2hd0g6a-treefmt-check.drv^*"
2025-03-13T16:57:24.562771Z DEBUG flake_iter::cmd::build: Building derivation 3 of 4 drv="/nix/store/3lp1hbdxc5p8dl464l2crj6hhq4b77ng-nix-shell.drv^*"
2025-03-13T16:57:25.819596Z DEBUG flake_iter::cmd::build: Building derivation 4 of 4 drv="/nix/store/5rv7zx5vawdjscj43g4s5gbihv4qylmh-treefmt.drv^*"

Would it make sense to highlight/expose this on GitHub Actions as a job summary?

I was thinking of something like this:

	x86_64-linux	aarch64-linux
foo.drv	✅	✅
bar.drv	🛑	✅

Bonus would be to also include the flake-schema info, if supplied.

Happy to take a swing at this, just wanted to reach out first to get a sense what the vision for GHA feedback to flake-iter build is in general.

How I think I'd go about this:

1. teach flake-iter build to output a form of the above summary to a markdown file (or the $GITHUB_SUMMARY_STEP env var on GHA).
2. ... I think there is no step two? The summary should already be rendered ...

(Caveat: not sure whether it would be easily possible to create something like the above table already, but at least some markdown output in that direction).

[lucperkins]

We generate Markdown for GitHub Actions with the Flake Checker so there's some reasonable prior art here. If it existed at the time I probably would've used Askama instead. The current Handlebars template we use: https://github.com/DeterminateSystems/flake-checker/blob/main/src/templates/summary.standard.md.hbs.