pretix: fix pretix-manage runserver by using makeWrapper

I noticed that running pretix runserver does not work:

% nix run .#pretix -- runserver localhost:8001
  File "/nix/store/ack7nv8s8g5lcmypr9ayb5291n0yk4ad-pretix-2025.5.0/bin/pretix-manage", line 2
    PATH=${PATH:+':'$PATH':'}
         ^
SyntaxError: invalid syntax

I don’t know exactly how or why I get a Python error when
bin/pretix-manage clearly has a Bash shebang, but I looked
at other django web apps in nixpkgs and noticed that they
were using a different way to create the manage.py wrapper.
And indeed, when using makeWrapper, runserver starts working.

pkgs/by-name/pr/pretix/package.nix

@@ -222,7 +222,9 @@ python.pkgs.buildPythonApplication rec {
 
   postInstall = ''
     mkdir -p $out/bin
-    cp ./src/manage.py $out/bin/pretix-manage
+    cp ./src/manage.py $out/${python.sitePackages}/pretix/manage.py
+    makeWrapper $out/${python.sitePackages}/pretix/manage.py $out/bin/pretix-manage \
+      --prefix PYTHONPATH : "$PYTHONPATH"
 
     # Trim packages size
     rm -rfv $out/${python.sitePackages}/pretix/static.dist/node_prefix