python: execute pythonImportsCheckPhase in subshell

Execute in subshell so that the cwd returns back
to the original directory. This is important as
it enables pythonImportsCheck to work with checkPhase

pkgs/development/interpreters/python/hooks/python-imports-check-hook.sh

@@ -6,7 +6,7 @@ pythonImportsCheckPhase () {
 
     if [ -n "$pythonImportsCheck" ]; then
         echo "Check whether the following modules can be imported: $pythonImportsCheck"
-        cd $out && eval "@pythonCheckInterpreter@ -c 'import os; import importlib; list(map(lambda mod: importlib.import_module(mod), os.environ[\"pythonImportsCheck\"].split()))'"
+        ( cd $out && eval "@pythonCheckInterpreter@ -c 'import os; import importlib; list(map(lambda mod: importlib.import_module(mod), os.environ[\"pythonImportsCheck\"].split()))'" )
     fi
 }