Allow bundle to handle an evaluated nix expression

Example (note the --arg vs --argstr for the package argument):
nix-build appimage-bundle.nix --arg package 'with import <nixpkgs>{}; writers.writePython3Bin "helloThere.py" {} "print(1)\n"' --argstr exec helloThere.py

appimage-bundle.nix

@@ -1,5 +1,7 @@
 # use like this:
 # nix-build appimage-bundle.nix --argstr package hello --argstr exec hello
+# nix-build appimage-bundle.nix --arg package 'with import <nixpkgs>{}; writers.writePython3Bin "helloThere.py" {} "print(1)\n"' --argstr exec helloThere.py
+
 
 {nixpkgs ? import <nixpkgs>{}, 
 package,
@@ -15,18 +17,19 @@ let
       };
       src = env;
       inherit exec;
-      buildInputs = [ drv nixpkgs.coreutils nixpkgs.gnutar nixpkgs.xz ];
+      buildInputs = [ drv ];
       usr_fonts = buildEnv {
         name = "fonts";
         paths = [noto-fonts];
       };
       buildCommand = ''
         source $stdenv/setup
-        mkdir -p $out/bin
-        cp -rL ${env}/* $out/
-        chmod +w -R $out/
-
+        mkdir $out
+        shopt -s extglob
+        ln -s ${env}/!(share) $out/
         mkdir -p $out/share/fonts
+        ln -s ${env}/share/* $out/share/
+
         cp ${usr_fonts}/share/fonts/* $out/share/fonts -R
 
         mkdir -p $out/share/icons
@@ -50,8 +53,18 @@ let
   };
 
 in
+  let results = 
+      if (nixpkgs.lib.isDerivation package && !(nixpkgs.lib.isString package))
+      then { 
+          name = package.name;
+          target = appimage_src package "${exec}";
+          extraTargets = [];
+        }
+        else { 
+          name = nixpkgs."${package}".name;
+          target = appimage_src (nixpkgs."${package}") "${exec}";
+          extraTargets = [];
+        };
+  in
   with (import (./appimage-top.nix){nixpkgs' = nixpkgs.path;});
-    (appimage (appdir {
-      name = package;
-      target = appimage_src nixpkgs."${package}" "${exec}";
-    })).overrideAttrs (old: {name = package;})
+  (appimage (appdir results )).overrideAttrs (old: {name = results.name;})