installation: virtualenv based path for static

Relative path fails if you're using symlinks for `src/invenio` or want to use a different directory layout.

It also kind of matches the way the static path is built: https://github.com/jirikuncar/invenio/blob/d9fcead025afa7653cfabaedea563a4b24a6c1c8/invenio/base/factory.py#L108-L111