Modify $wgUser during the upload media file job

Abusefilter still looks at $wgUser during the UploadVerifyFile
hook. I wouldn't be surprised if other things in MW still
rely on $wgUser.

This was causing Special:Abuselog to list all gwtoolset uploads
as coming from localhost.

Bug: 67504
Change-Id: I223d24d9540bb13445977855979e64e31f581f90