xonsh: add PYTHONPATH

pkgs/shells/xonsh/default.nix

@@ -29,6 +29,19 @@ python3Packages.buildPythonApplication rec {
     find scripts -name 'xonsh*' -exec sed -i -e "s|env -S|env|" {} \;
     find -name "*.xsh" | xargs sed -ie 's|/usr/bin/env|${coreutils}/bin/env|'
     patchShebangs .
+
+    substituteInPlace scripts/xon.sh \
+      --replace 'python' "${python3Packages.python}/bin/python"
+
+  '';
+
+  makeWrapperArgs = [
+    "--prefix PYTHONPATH : ${placeholder "out"}/lib/${python3Packages.python.libPrefix}/site-packages"
+  ];
+
+  postInstall = ''
+    wrapProgram $out/bin/xon.sh \
+      $makeWrapperArgs
   '';
 
   disabledTests = [