Fix self-assign fast path

[Kobzol]

We were checking the author of the PR, rather than the author of the comment.

Fixes: #2058

[jieyouxu]

Looks good, I don't work on the triagebot codebase tho 😄

[jieyouxu]

r? triagebot

[Urgau]

Shouln't we always use event.user()?

[Kobzol]

It should be the same, because the determine_assignee is only called on PR being opened or converted from draft. It seemed more explicit like this, because event.user().login always confuses me a bit. But if you prefer the user() function, I'll switch it.

[Urgau]

I don't have a preference. Let's merge as is.