Installer: properly override default $wgLogo value

Setup.php uses the value of $wgScriptPath in $wgLogo before the
installer determines the former, so it is necessary to override
the latter even before the WebInstaller_Options page is shown
(because it might never be shown).

Bug: 48084
Change-Id: Icc577c1bb1f78adc7a7cfb0581ae8ea2facba4cf
Merge "Installer: properly override default $wgLogo value"