Restore original librarian-chef install path

setup! changes the librarian-chef install path (env.config_db.local['path']) to a temp directory for the duration of the test run.  Teardown removes the temp directory but does not reset the librarian-chef install path.  One result is that, after running chefspec, "librarian-chef install" will install to the previously-used temp directory and the intended cookbook location ("cookbooks" by default) becomes out of date and useless.

Fixed by simply remembering the original librarian-chef install path and restoring it on teardown.