ci: Generate MinIO credentials within LAVA jobs

As these credentials are valid only for 15 minutes, generate them closer
to when they are going to be used.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6124>
5 files changed