ci: Move common variables out into a separate file

ci: Move common variables out into a separate file

Moving common variables out allows for other projects like virglrenderer
to be able to reuse Mesa CI's containers

Signed-off-by: Rohan Garg <rohan.garg@collabora.com>
Reviewed-by: Emma Anholt <emma@anholt.net>
[cristian: fixed conflicts while rebasing on latest main; updated tags]

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14413>