ci: Use half as many parallel softpipe / virgl test jobs

We're now using at least twice as many CPU cores per job (on shared
runners), so they only take about half as long, and should still be
under 10 minutes.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6011>
1 file changed