build: xvfb-run call from meson fails.

Note that I first thought to use the full option name (`--auto-display`)
but I just got the same error again. It seems xvfb-run from Debian
bookworm just doesn't have this option, and only the older option
--auto-servernum (which is said to be deprecated in favor of
--auto-display in Fedora's xvfb-run for instance, but apparently not on
Debian). At least this works on all distributions (or so it would seem).

Fixes:

> xvfb-run: invalid option -- 'd'

build/meson/run_test_env.sh

@@ -11,7 +11,7 @@ if [ -n "${UI_TEST}" ]; then
   # Also use dbus-run-session to make sure parallel tests aren't failing
   # as they simultaneously try to own the "org.gimp.GIMP.UI" D-Bus name
 
-  xvfb-run -d --server-args="-screen 0 1280x1024x24" \
+  xvfb-run --auto-servernum --server-args="-screen 0 1280x1024x24" \
     dbus-run-session -- "$@"
 
 else