tests/docker: move sh4 to use debian-legacy-test-cross

sh4 is another target which doesn't work with bookworm compilers. To
keep on buster move across to the debian-legacy-test-cross image and
update accordingly.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20231030135715.800164-1-alex.bennee@linaro.org>