Fix broken setup_doc_virtualenv target.

The sentinel file .setup_virtualenv path was different between the
target name and what was actually generated, so tracking of dependency
timestamps was broken.
PTFM-6201 Sanitize initialization of virtualenv.
Force rebuild of _modules so it doesn't get stale.
Bumped R bindings version to 0.38.1
Changelog update for v0.38.1