Install: Allow an installation to go through with a custom user table with the username 'admin'.

This could be better, but this solves the regression caused by [25115].

fixes #27093, see #24078.

Built from https://develop.svn.wordpress.org/trunk@27822


git-svn-id: http://core.svn.wordpress.org/trunk@27656 1a063a9b-81f0-0310-95a4-ce76da25c4cd