weechatScripts.weechat-matrix: fix PYTHONPATH

matrix.py tries to import its dependencies, and its own code from
$out/${python.sitePackages}, and neither of these worked because
nothing was telling Python where to load that stuff from.

Using patchPythonScript made it so matrix.py could find its
dependencies, but it still wasn't able to find its own code, so that
had to be added to program_PYTHONPATH beforehand so patchPythonScript
would include it as well.

Fixes: https://github.com/NixOS/nixpkgs/issues/89499

pkgs/applications/networking/irc/weechat/scripts/weechat-matrix/default.nix

@@ -70,6 +70,10 @@ in buildPythonPackage {
   '';
 
   dontPatchShebangs = true;
+  postFixup = ''
+    addToSearchPath program_PYTHONPATH $out/${python.sitePackages}
+    patchPythonScript $out/share/matrix.py
+  '';
 
   meta = with lib; {
     description = "A Python plugin for Weechat that lets Weechat communicate over the Matrix protocol";