installer: Move Server/Extension/Path preparations out of envChecks

These aren't checks but preparations, they should always run
even when the user is further in the installer process or when
the server is generating the WebInstaller stylesheet (which needs
wgScriptPath to be set in order to remap image paths correctly).

Bug: 67810
Change-Id: I98eb15096ff4ef3e464b9d00c58b036a608deb41
Merge "installer: Move Server/Extension/Path preparations out of envChecks"