gitlab-ci: use xvfb -noreset

Drop the gratuitous wait and use the -noreset Xvfb option. This should
resolve the intermittent fallouts that we're seeing.

Huge thanks goes to Pekka and Daniel for pointing it out.

Closes: mesa/waffle#70
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
6 jobs for xvfb-noreset in 2 minutes and 11 seconds
Status Job ID Name Coverage
  Container
passed container #11770754

00:00:16

 
  Build
passed cmake #11770756

00:00:28

passed cmake-mingw #11770757

00:01:14

passed meson #11771375

00:00:35

passed meson #11770755

00:00:32

passed meson #11770780

00:00:47