installer: Fix "relation 'user' does not exist" error for Postgres

installer: Fix "relation 'user' does not exist" error for Postgres

On Travis CI, the Postgres build has been failing very early on
in the installer (before phpunit) due to a database error:

> Creating administrator user account.. DBQueryError at Database.php:1059
> Query: SELECT user_id FROM "user" WHERE user_name = 'Admin' LIMIT 1
> Function: User::idForName
> Error: 42P01 ERROR:  relation "user" does not exist
> LINE 1: SELECT /* User::idForName  */ user_id FROM "user" ...

This is because the installer makes its own Database object without
involving ServiceWiring or MWLBFactory, which means wgDBport and
(more importantly) 'keywordTableMap' don't get applied.

While keywordTableMap doesn't matter during the database installation,
after the installer is done updating GlobalVarConfig and resetting
MediaWikiServices, DatabaseInstaller::enableLB takes that homemade
connection and injects it into MediaWikiServices by redefining
the 'DBLoadBalancerFactory' service. Which then affects all use
of wfGetDB(), such as from User::idForName().

Bug: T30162
Bug: T75174
Bug: T75176
Change-Id: I7af58c4beffc4908a93c0c1d8ab1aec9d4ec57c6