ci: Don't build gnome-shell's man pages

One of the man pages is now generated using asciidoc, which is missing
from the CI image. But given that this doesn't depend on mutter in any
way, just disable man pages in the gnome-shell build instead of updating
the Dockerfile.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/740

.gitlab-ci.yml

@@ -72,7 +72,7 @@ can-build-gnome-shell:
     - meson install --no-rebuild -C build
   script:
     - .gitlab-ci/checkout-gnome-shell.sh
-    - meson gnome-shell gnome-shell/build --prefix /usr
+    - meson gnome-shell gnome-shell/build --prefix /usr -Dman=false
     - ninja -C gnome-shell/build install
   only:
     - merge_requests