ci: Test ninja dist in meson jobs

While this isn't critical yet, it might become so at some point, so
let's make sure it keeps working.

Don't set the PIGLIT/XTEST_DIR variables for ninja dist, testing XTS
once against each DDX is enough (as part of ninja test).

Requires git in the docker image.

v2:
* Drop /usr/local/bin/xkbcomp symlink, no longer needed with xkbcomp
  1.4.1 (Peter Hutterer)
4 jobs for master in 11 minutes and 5 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Docker Image
passed #6819289
debian-buster

00:06:23

 
  Build And Test
passed #6819290
autotools

00:04:41

passed #6819291
meson

00:01:57

passed #6819292
meson-noglamor

00:01:57