static-nix-shell.mkZsh: implement shellPreamble to reduce wrapping

pkgs/by-name/static-nix-shell/package.nix

@@ -271,9 +271,16 @@ in rec {
       pkgsEnv = [ zsh ] ++ (builtins.attrValues pkgsAsAttrs);
       pkgExprs = insertTopo "zsh" (builtins.attrNames pkgsAsAttrs);
     in mkShell ({
-      # TODO: generate `shellPreamble` here to reduce wrapping
       inherit pkgsEnv pkgExprs;
       interpreter = lib.getExe zsh;
+      postConfigure = ''
+        shellPreamble='
+        export PATH=''${PATH:+$PATH:}'"$append_PATH"'
+        export XDG_DATA_DIRS=''${XDG_DATA_DIRS:+$XDG_DATA_DIRS:}'"$append_XDG_DATA_DIRS"'
+        '
+        unset append_PATH
+        unset append_XDG_DATA_DIRS
+      '';
     } // (removeAttrs attrs [ "pkgs" "zsh" ])
   );