ci: Change update-mutter to also install to sysext dir

Out development toolbox contains an `update-mutter` command to
make it easier to keep mutter up-to-date for people who only
want to deal with gnome-shell.

To allow them to make use of the new sysext tooling, change the
command to install both to the toolbox root and the sysext
directory.

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

.gitlab-ci/build-toolbox-image.sh

@@ -57,7 +57,10 @@ build_container() {
   local update_mutter=$(mktemp)
   cat > $update_mutter <<-EOF
 	#!/bin/sh
-	/usr/libexec/install-meson-project.sh https://gitlab.gnome.org/GNOME/mutter.git $MUTTER_BRANCH
+	TOOLBOX=\$(. /run/.containerenv; echo \$name)
+	/usr/libexec/install-meson-project.sh \\
+	  --destdir=/ --destdir=/var/lib/extensions/\$TOOLBOX \\
+	  https://gitlab.gnome.org/GNOME/mutter.git $MUTTER_BRANCH
 	EOF
   buildah copy --chmod 755 $build_cntr $update_mutter /usr/bin/update-mutter