python3Packages.weasyprint: restore overriding of fontconfig file

This builds on the fix for Darwin from
https://github.com/NixOS/nixpkgs/pull/207348, but letting the variable
be configurable in each environment. This avoids needing to specialize
and rebuild the derivation just to alter the fontconfig.

pkgs/development/python-modules/weasyprint/default.nix

···
 
   FONTCONFIG_FILE = "${fontconfig.out}/etc/fonts/fonts.conf";
 
-  # Fontconfig error: Cannot load default config file: No such file: (null)
+  # Set env variable explicitly for Darwin, but allow overriding when invoking directly
   makeWrapperArgs = [
-    "--set FONTCONFIG_FILE ${FONTCONFIG_FILE}"
+    "--set-default FONTCONFIG_FILE ${FONTCONFIG_FILE}"
   ];
 
   postPatch = ''