tools/toolbox: Install sysext dependencies if necessary

While the toolbox image itself has all necessary dependencies to
build and run the shell, we still need them in the destdir when
building a system extension.

Achieve this by running .gitlab-ci/install-common-dependencies.sh
if it exists, which is the script that is used both for gnome-os
system extensions and mutter's CI image.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3498>

tools/toolbox/meson-build.sh

@@ -52,6 +52,12 @@ compile_command() {
 
 install_command() {
   local destdir=${BUILD_SYSEXT:+/var/lib/extensions/$TOOLBOX}
+
+  local install_deps=.gitlab-ci/install-common-dependencies.sh
+  if [[ $BUILD_SYSEXT && -x $install_deps ]]; then
+    echo -n "$install_deps --destdir $destdir && "
+  fi
+
   if [[ $destdir || ! $RUN_DIST ]]; then
     echo -n "sudo meson install -C $BUILD_DIR ${destdir:+--destdir=$destdir}"
   else