Use ${TMPDIR} as the default working directory.

This patch uses ${TMPDIR} as the default working directory, falling back
to the old behaviour is /tmp was not set. This also passes the desired
working directory to delta_generator as a TMPDIR env variable.

Bug: 34250932
Test: `strace -e trace=open -f -- brillo_update_payload generate --payload pay.bin --target_image tf.zip --work_dir /tmp/foobar 2>&1 | grep tmp` only shows access to /tmp/foobar, not /tmp
Change-Id: I4972a4010d7eed73b2fd1cc9c782b0775509b50c