ci: Improve interrupt signal handling in crosvm-runner.sh

ci: Improve interrupt signal handling in crosvm-runner.sh

Run crosvm as a background process in order to allow intercepting
interrupt signals (INT, TERM) and properly release/cleanup any allocated
resources.

This is particularly helpful when one or more crosvm tasks hang, which
will eventually prevent subsequent instances to be started - currently
we can handle up to 128 concurrent crosvm instances per runner.

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/15238>