python.pkgs.pytest: fix setupHook .pytest-cache

In 610485faa7 a setupHook was added to `pytest`
to prevent the creation of `.pytest-cache` folder. Some pytest plugins relied
on the cache and were thus failing.

This commit permits the cache during build time but removes it in a `postFixupHook`.

pkgs/development/python-modules/pytest/default.nix

@@ -27,9 +27,11 @@ buildPythonPackage rec {
     runHook postCheck
   '';
 
-  # Don't create .pytest-cache when using py.test in a Nix build
+  # Remove .pytest-cache when using py.test in a Nix build
   setupHook = writeText "pytest-hook" ''
-    export PYTEST_ADDOPTS="-p no:cacheprovider"
+    postFixupHooks+=(
+        'find $out -name .pytest-cache -type d -exec rm -rf {} +'
+    )
   '';
 
   meta = with stdenv.lib; {