Add missing execution on archived task query

Summary: Fixes T8599. I'm not sure how to reproduce the original issue, but I'm fairly confident that the issue is that the issue is that execute() is not called on the query object.

Test Plan: Created a Harbormaster build plan with a single "Lease Host" step. Ran ./bin/harbormaster build --plan 1 D1 from the command line and hit the exception as described in T8599. Applied patch and hit a different exception (which I think is just because I don't know how to use Drydock and Harbormaster).

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: hach-que, epriestley, Korvin

Maniphest Tasks: T8599

Differential Revision: https://secure.phabricator.com/D13335