radv/ci: increase the parallelism of the vkcts-navi21 job

The vkcts-navi21 job currently take ~13 minutes each, but we are about
to start running the VK_EXT_shader_object tests which should
significantly increase our runtime and thus would exceed the 15 minutes.

Signed-off-by: Martin Roukala (né Peres) <martin.roukala@mupuf.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25325>