Set download_tmp_dir explicitly to be a directory under the output directory.

This directory is where outputs are download first before being moved
to their final destination to ensure atomicity.

Fixes: 307322068
Test: m # when out is located in a different partition

(cherry picked from commit bc061763 - AOSP/main)

Merged-In: Ic224bf21c07566de00d292d02f1c0f7f727dcb08
Change-Id: Ic224bf21c07566de00d292d02f1c0f7f727dcb08