Fix flaky test TestPipeline/context_done

TestPipeline/context_done is testing that the context cancellations are respected when waiting for a job to be picked up by a worker. Unfortunately the test is flaky as the select statement where we are waiting for a job to be picked up or the context being canceled is indeterministic. This commit fixes the issue by overriding the channel the job is being sent to with a channel where it will never be received from.

Closes #5792 (closed)