Restore previous behavior of `get_build_var TMPDIR`

My previous change makes TMPDIR a temporary directory that's removed
before we exit. So instead of starting ckati to get this value to a
directory that no longer exists, return the semi-stable value that we
use during a normal build.

Fixes: 142277335
Test: get_build_var TMPDIR
Change-Id: I37d1219b1fda09f131dee17ef7b91e475d3e39ad