synfigstudio: fix chroot build / hydra

The error message was:
Fontconfig error: Cannot load default config file
make[2]: *** [128x128/synfig_icon.png] Segmentation fault

(cherry picked from commit 759980c4fe9b07932f671d2e657189068d117405)

Conflicts:
	pkgs/top-level/all-packages.nix