Add python and tox to flake.nix devshell (#6233)

Without tox any python `scripts/make-python-env.sh` does not run. Maybe
at some point we can even generate the environment for testing with
`venvHook` like functionality. We could also add `python3` instead of
fixing it to 3.11, but I don't know which version core expects

flake.nix

@@ -584,6 +584,9 @@
               cargo-nextest
               perl # needed to build vendored OpenSSL
               git-cliff
+              (python3.withPackages (pypkgs: with pypkgs; [
+                tox
+              ]))
             ];
           };
       }