wrapAppImage: default `extraPkgs` to `pkgs: [ ]` Sane default in preparation for the next commit.

pkgs/build-support/appimage/default.nix

···
 
   wrapAppImage = args@{
     src,
-    extraPkgs,
+    extraPkgs ? pkgs: [ ],
     meta ? {},
     ...
   }: buildFHSEnv