This reverts commit e28916c9 (in general).
The optimization introduced by that change is no longer necessary following commit b8d962a9 ("grid/slapgrid.py: reduce number of installation.available calls"). Since API calls are now less frequent, avoiding the POST to the master offers little benefit.
Worse, skipping the POST can now cause issues: if the software is reconstructed and its state transitions to "building", it may never be reported back as "available" in some race conditions, leading to incorrect or outdated local state.