buildman: Fix CROSS_COMPILE output for sandbox

The previous attempt at fixing this broke the normal usage of the -A
flag.

At present, 'buildman -A sandbox' adds the path containing the
toolchain. We can assume that this is in the path and we don't want to
set CROSS_COMPILE=/bin/

Change this to align with what MakeEnvironment() does, but only for
sandbox boards.

Signed-off-by: Simon Glass <sjg@chromium.org>