ci: the ci is broken, it fails to generate the documentation on pages
$ tox -e doc
doc create: /builds/cubicweb/cubicweb/.tox/doc
ERROR: cowardly refusing to delete `envdir` (it does not look like a virtualenv): /builds/cubicweb/cubicweb/.tox/doc
$ tox -e doc
doc create: /builds/cubicweb/cubicweb/.tox/doc
ERROR: cowardly refusing to delete `envdir` (it does not look like a virtualenv): /builds/cubicweb/cubicweb/.tox/doc