ci: Use sudo to run meson install

ci: Use sudo to run meson install

Meson stopped using polkit for automatic privilege elevation, and
will no longer attempt any priviledge elevation when not running
interactively.

Running the entire install command as root used to be problematic
in the past, as it could result in ownership changes of files in
the build directory that would result in build failures later,
but the aforementioned change leaves us with little choice.

Apparently `meson install` will spawn `ninja` with dropped
privileges when a rebuild is needed, so hopefully this will
no longer be an issue.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2892>