@GrahamcOfBorg build mdbook

@worldofpeace I actually did have this script posting that comment automatically in the previous iteration, but per comment from @grahamc himself it's not necessary!

I think posting it kicks the builder to start building instantly, while otherwise it should eventually come through and do it on its own. Let's wait and see on #80399, which has currently been open for 10 minutes with no OfBorg signs of life . . .

EDIT: It did eventually wake up! It took about 25 minutes; I'm guessing the comment is the difference between push-based triggering and poll-based triggering, with the latter only happening when OfBorg is actually ready to start processing.

ofborg's builds aren't poll-based, the builds are triggered once the evaluations complete.

Side note: it used to be required to trigger the builds for any PR where the person opening the PR wasn't pre-approved. ofborg now automatically builds all PRs, no matter who opens them. The only remaining difference is whether or not they are triggered on darwin.

Right, but the "Some checks haven't completed yet" (or any status/signs of life at all) didn't pop up for ~25 minutes. It'd be nice if ofborg immediately set the status as pending on PR opens, so those impatient humans don't try to poke it unnecessarily!

(If you post the comment, it instantly sets the status to pending, at which point you know to just sit tight.)