Use GitHub's workflow message feature to present errors on the PR itself

[philiptaron]

The new nixf-tidy provides a great example of how nixpkgs-check-by-name can level up. It uses GitHub’s workflow message feature to render error messages in the ‘Files changed’ overview, directly below the lines with errors.

Check an example here: workflows/check-nixf-tidy.yml: by Aleksanaa · Pull Request #1 · Aleksanaa/nixpkgs · GitHub 41.

We should do the same thing when run from CI. The current output is great for terminal use cases; this output is better for CI.

[infinisil]

I've considered this before, but a problem is that such errors are limited to a single line, which then kind of requires either making two separate ways of printing every error, or not using multi-line errors in the terminal too. I don't have a strong opinion on this though. If it can be made to work well, I'm all for this :)

[philiptaron]

a problem is that such errors are limited to a single line

The schema seems to indicate that it's not: it contains both line and endLine.

[infinisil]

I mean that the error message itself is limited to only being a single line. E.g.

echo -e "::error file=app.js,line=1,col=5,endColumn=7::Missing semicolon\nflorp"

Probably just annotates "Missing semicolon", while "florp" is on its own log line, not attached to any lines of code