appimageTools.extract: add postExtract option (#261190)

pkgs/build-support/appimage/default.nix

···
     ];
   };
 
-  extract = args@{ name ? "${args.pname}-${args.version}", src, ... }: pkgs.runCommand "${name}-extracted" {
+  extract = args@{ name ? "${args.pname}-${args.version}", postExtract ? "", src, ... }: pkgs.runCommand "${name}-extracted" {
       buildInputs = [ appimage-exec ];
     } ''
       appimage-exec.sh -x $out ${src}
+      ${postExtract}
     '';
 
   # for compatibility, deprecated