pre-commit: prevent propagating inputs

Fixes a bug where pre-commit would pollute the PYTHONPATH with its
dependencies, potentially interfering with the user's dependencies.

pkgs/tools/misc/pre-commit/default.nix

@@ -101,6 +101,12 @@ buildPythonApplication rec {
     deactivate
   '';
 
+  # Propagating dependencies leaks them through $PYTHONPATH which causes issues
+  # when used in nix-shell.
+  postFixup = ''
+    rm $out/nix-support/propagated-build-inputs
+  '';
+
   disabledTests = [
     # ERROR: The install method you used for conda--probably either `pip install conda`
     # or `easy_install conda`--is not compatible with using conda as an application.