Use our own TMPDIR

Some people have run into issues where /tmp is on a smaller partition
than their main source tree, and if it fills up (or only has a few GB
free), their builds can start failing.

There may also be a subset of people whose /tmp may be slower than their
out directory -- if they put their src and out trees on a SSD, but keep
/tmp on a spinning HDD.

Bug: 71755844
Test: check out/soong.log for TMPDIR
Test: do a build, watch out/soong/.temp
Change-Id: Ib749ec736ad0f6c8ce7453861a91b3e99ddbd92d