gitlab-ci: add qemu-system-arm to the existing tests-artifacts container

gitlab-ci: add qemu-system-arm to the existing tests-artifacts container

Add qemu-system-arm to the existing test-artifacts qemu container (which
doesn't get build for every iteration but only updated once in a while.)

With qemu-system-arm available, we'll be able to run ARM32 tests.

This patch also bumps the QEMU version to v6.0.0 for both arm32 and
arm64 (the test-artifacts container is one, shared for both).

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>