Add workaround for Ubuntu 23.04 putting 'local' in the Python platbase path.

Ubuntu 23.04 has messed up the Python internal path reporting, by sticking the local from e.g. /usr/local into the Python part of the path rather than the prefix. (Maybe this is intentional based on the build being made by root, or something? No hints online.) Assuming local/local structures in the Python path are never intentional, this hack strips one of them back out again.