[shipit] Cancelling "promote" didn't cancel the underlying tasks

[mitchhentges]

This might be related to #2226.
When I had double-promoted 69.0b9, I cancelled it so that I could do a fresh build.
Though the second promote was erroneous, I still expected one of the graph's tasks to be cancelled when I cancelled the promotion.

However, ship-it didn't cancel either graph when the promotion was cancelled.

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I don't understand the bounds of this issue, but I'm reporting it so that we have context if we see the issue again