make attachment_fu write to just /tmp

this makes it so that any user would be allowed to
create a tmp file in console rather than just the
owner of canvas

refs CNVS-15701

test plan
 - in console should be able to create a temp file
   a.create_temp_file
 - create a profile pic
 - run DataFixup::RegenerateUserThumbnails to
   delete and re-generate profile pics
 - it should work

Change-Id: I880d2cc88db00d0bae9dcfabf4c81f6a393911ee
Reviewed-on: https://gerrit.instructure.com/41725
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brad Horrocks <bhorrocks@instructure.com>
Reviewed-by: Cameron Matheson <cameron@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>