gitlab/azure: Use the -w option for sandbox_spl

Avoid needing to know about the internal .bm-work directory, by passing
the -w flag to buildman.

This does not affect travis since the previous commit already used the -w
flag.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>