Correct behavior of "writable" Almanac service binding for repository services

Summary: Ref T13611. This property worked correctly when implemented in D19357. The behavior was broken by D20775, which tested node-level routing but did not specifically re-test the "writable" property. This was difficult to spot because ref query outcomes weren't observable in the UI, and the ref itself had the correct property value.

Test Plan:
See D21575. After this change, the UI shows the correct state, rather than showing a read-only service ref as writable:

Maniphest Tasks: T13611

Differential Revision: https://secure.phabricator.com/D21576