tests/docker: convert riscv64-cross to lcitool

tests/docker: convert riscv64-cross to lcitool

We still need to base this on Debian Sid until riscv64 is promoted to
a release architecture (or another distro provides a full cross
compile target). We use the new qemu-minimal project description to
avoid bringing in all the extra dependencies because every extra
package is another chance for sid to fail.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230630180423.558337-16-alex.bennee@linaro.org>