We need to move all maintenance scripts to use PHP 7.2 instead of HHVM.
This can be done by simply prepending `PHP=php7.2` to all the invokations of mwscript etc in the maintenance scripts.
What needs to be done in detail:
 Separate production maintenance scripts from wikitech ones*
 Pick a few scripts to migrate first, notify the owners, migrate
 Migrate the wikidata dispatcher
 Migrate all other scripts by setting php7.2 as the preferred debian alternative
 Revert all the modifications to the scripts above as we don't need to force
* Some cronjobs are shared with wikitech, which has not been switched to PHP7. To avoid blocking this task we can keep production and wikitech maint jobs separate
Related to: T219135: