Build: hardcode the Docker username for now (#10172)
* Build: hardcode the Docker username for now
This is a quick fix for a hotfix.
`RTD_DOCKER_USER` is `docs:docs` so we can't use it inside the path.
* Build: deal with PATH on old Docker images (stable, latest, testing)
These old images are using `pyenv` instead of `asdf`, so we need to use some
different paths to find the binaries.
* Lint
