Hmm, I would not expect environment.variables.GDK_PIXBUF_MODULE_FILE = "$(echo ${pkgs.librsvg.out}/lib/gdk-pixbuf-2.0/*/loaders.cache)"; to
work at all, but apparently the variables are not escaped at all:
https://github.com/NixOS/nixpkgs/blob/5bae6dea205144bbf2689a42fdbbf490745d52f4/nixos/modules/config/shells-environment.nix#L12-L28;
I do not see any relevant changes in the module either.
It would be much better to use the new gdk-pixbuf module from unstable:
services.xserver.gdk-pixbuf.modulePackages = [ pkgs.librsvg ]; You
can cherry-pick Use a NixOS module for generating the gdk-pixbuf loaders cache. by ambrop72 · Pull Request #42562 · NixOS/nixpkgs · GitHub