shell.nix: Define gdk pixbuf loaders from build dependencies.

shell.nix: Define gdk pixbuf loaders from build dependencies.

This resolves a problem when the wrapped Factor binary is executed with
`GDK_PIXBUF_MODULE_FILE` set to an incompatible version.  This happens if e.g.
Factor is run from within an Emacs that uses newer gdk versions.

Related NixOS issue: https://github.com/NixOS/nixpkgs/issues/54278

In the long run, this should be replaced by a derivation which correctly runs
through all the phases correctly to set up `wrapGAppsHook` correctly.  Even
then, the correct behavior would probably be to have the surrounding
environment (e.g. Emacs) restore the default variable value in it's
sub-environments.