tests/docker: use debian-all-test-cross for m68k

Maintaining two sets of containers for test building is silly. While
it makes sense for the QEMU cross-compile targets to have their own
fat containers built by lcitool we might as well merge the other
random debian based compilers into the same one used on gitlab.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231029145033.592566-11-alex.bennee@linaro.org>