Fix bloat check workflow concurrency

[bzbarsky-apple]

Problem

We have not had a completed bloat check in 13 hours; new ones keep canceling old ones.

Change overview

Make new bloat checks not cancel old ones, at least if the old ones are running already.

Testing

Will see if it helps when it merges... Hard to test otherwise.

[bzbarsky-apple]

Doesn't this mean we can

Can which?

I'm still concerned that a newly scheduled bloat check might cancel a previously-pending one and hence lose its place in the queue... Not sure how that works, exactly.

[msandstedt]

Doesn't this mean we can

Can which?

I'm still concerned that a newly scheduled bloat check might cancel a previously-pending one and hence lose its place in the queue... Not sure how that works, exactly.

Sorry, this was an inadvertent comment. I was typing while looking at the github docs.

But yes, by my reading new jobs will cancel previously-pending jobs. But these will no longer cancel the running job. This seems like it is what we want. We still have the maximum queue depth of 1, but ensure that the running job always completes.