Don't let bin/drydock lease --attributes overwrite blueprints

Summary:
Depends on D19076. Ref T13073. Blueprints are stored as an attribute and setAttributes() overwrites all attributes.

This is sorta junk but make it less obviously broken, at least.

Test Plan: Ran bin/drydock lease --type working-copy --attributes x=y without instantly getting a fatal about "no blueprint PHIDs".

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13073

Differential Revision: https://secure.phabricator.com/D19077