The mutter image contains a custom gdk-pixbuf build without the
glycin loader, in order to unbreak running installed tests in CI.
The downside is that we end up with no SVG loader. For the toolbox
image we don't care about installed tests, but we very much care about
working icons.
So reinstall the distro gdk-pixbuf package to bring back the glycin
loader (and thus SVG support).