Miss-islington should not self-assign PR that failed to backport

[Mariatta]

It should assign it back to the approver.

[Mariatta]

GitMate.io thinks possibly related issues are #7 (Trigger the backport by leaving comments to miss-islington), python/core-workflow#243 (Assign the failed PR to be backported to the core dev who merged it), #88 (Assign the failed PR to be backported to the core dev who merged it), #26 (Notify when backport PRs failed), and #94 (Approved backport PR was not automerged).

[AA-Turner]

She doesn't assign PRs to herself anymore, I don't think.