don't use write_attribute for gradebook importer temp storage

refs CNVS-15830

test plan:
 * download the gradebook, change a grade, and upload it again
 * it should work

Change-Id: I0fe245b23aa24152a2a62ad919cf045120ae1c47
Reviewed-on: https://gerrit.instructure.com/41974
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>