internal/postgres: take low-order part of instanceID
internal/postgres: take low-order part of instanceID

bugfix: take the end of the instance ID, which is random; not the beginning,
which seems fixed.

Also, given that we only have a handful of instances, 4 characters should suffice.

Change-Id: Ibc80f65e335fa2f3ebeb39322f85cb1a1a07a60e
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/556716
CI-Result: Cloud Build <ea5729d4cf3392b9d382b6ca272c65aa27554f44@system.gserviceaccount.com>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite