Correctly abort on interrupt instead of going into endless loop

Created by: mrnugget

Running :GoMetaLinter just pointed me to this "ineffective break statement" that I introduced in #328 with the following comment:

It also makes sure that we don't enqueue more tasks/steps after being canceled: that's the code in (*executor).Start() that aborts the Range call if the context has been cancelled/timed-out and aborts execution of a task if the goroutine already blocked on x.par.Acquire().

Well... turns out that wasn't the case, was it?

Instead the code only broke out of the select and continued trying to acquire a lock.

I also suspect that this bug might have something to do with a customer reporting that long-running containers are not stopped when hitting Ctrl-C (https://github.com/sourcegraph/sourcegraph/issues/16026) even after we thought the issue was fixed with #328. I'm not 100% sure on this one though.