Bump PHP memory limit from 800M to 1200M in populate_adm_tree.php

The PHP script populate_adm_tree.php was still failing with

PHP Fatal error:
Out of memory (allocated 552599552) (tried to allocate 32 bytes)
in populate_adm_tree.php on line 46

Increasing the memory limit is the obvious solution.
Testing on Toolforge, 800M was still not enough, 1200M was.

Follow-up to a230a6e6

Bug: T200337
Change-Id: Ie8b50ff7967e28f246f2954d8ed3984ec0c6fb74