Create summary comment for lintcheck runs

[Alexendoo]

Previously rust-lang/triagebot#1985

After a lintcheck run a comment will be created in the PR thread with an overview of the changes, example here Alexendoo#18 (comment) (plus the normal GHA debugging experience)

It will only comment if there are some changes, if there's already an existing comment it will be updated for each run

Similar to https://github.com/rust-lang/team/blob/master/.github/workflows/dry-run.yml

The PR number is supplied by the lintcheck run, so technically someone could forge it to be annoying and edit the summaries in other threads, but that is pretty low impact and easy to deal with. There is a pull_requests field on the event but as @Kobzol pointed out to me it's not populated for PRs from forks

r? @flip1995

changelog: none

[samueltardieu]

Great feature!

Since an existing comment will be edited, it might not be clear without also checking the status of the run (which might be far down if the PR has been re-pushed many times) that the lintcheck comment is up-to-date.

If this is not too complicated, it would be great if, at the beginning of a run, the existing lintcheck comment could be prefixed with a bold message saying something like "Results for the new lintcheck run should be available shortly". WDYT?

[Kobzol]

The comment being updated silently can be a bit opaque, so adding a note sounds like a good idea. I would suggest still doing the update rather than sending a new comment, though, as that would be super spammy :)

Workflow looks good, although I'm not sure if I understand the else branch in Create/update comment.

[Alexendoo]

If this is not too complicated, it would be great if, at the beginning of a run, the existing lintcheck comment could be prefixed with a bold message saying something like "Results for the new lintcheck run should be available shortly". WDYT?

Not easy unfortunately, since we're stashing the PR number in an artifact we don't learn it until after lintcheck has already concluded. If the pull_requests field were populated we could do it in a separate step when the workflow run begins

Workflow looks good, although I'm not sure if I understand the else branch in Create/update comment.

Hopefully made the comment clearer