python: make it possible to disable the python setup-hook

Without this it's impossible to use a python binary that depends on a
different python version than what's used during the current build.

pkgs/development/interpreters/python/setup-hook.sh

@@ -12,7 +12,9 @@ toPythonPath() {
     echo $result
 }
 
-addEnvHooks "$hostOffset" addPythonPath
+if [ -z "${dontAddPythonPath:-}" ]; then
+    addEnvHooks "$hostOffset" addPythonPath
+fi
 
 # Determinism: The interpreter is patched to write null timestamps when compiling python files.
 # This way python doesn't try to update them when we freeze timestamps in nix store.