$TMP, $DEPS, $SRCS, $OUT are no longer supplied by Buck in the
environment unless they appear in the command line. Pass $TMP
where it was assumed to be magically supplied. This allows steps
to continue to use buck-out/gen/ for temporary storage instead of
polluting the system /tmp.
Change-Id: Iea8380e5f93fa16ec77457eb76404832bde93a39