treewide: use pkgs.config instead of config.nixpkgs.config

`pkgs` may be passed in externally, in which case `config.nixpkgs.config` will not be set.

Follow-up to #257458.

nixos/modules/programs/firefox.nix

@@ -237,7 +237,7 @@ in
           # copy-pasted from the wrapper; TODO: figure out fix
           applicationName = cfg.package.binaryName or (lib.getName cfg.package);
 
-          nixpkgsConfig = config.nixpkgs.config.${applicationName} or {};
+          nixpkgsConfig = pkgs.config.${applicationName} or {};
           optionConfig = cfg.wrapperConfig;
           nmhConfig = {
             enableBrowserpass = nmh.browserpass;