Deprecate the environment variable USER for changing the user name of the RStudio user

[eitsupi]

Suggested by #489 (comment)

This process is buggy, so I think it make sense to remove it.
If we want to change the user name, just write a Dockerfile like this

FROM rocker/rstudio:4.2.0
ENV DEFAULT_USER=new_user
RUN usermod -l "$DEFAULT_USER" rstudio \
    && groupmod -n "$DEFAULT_USER" rstudio \
    && usermod -d /home/"$DEFAULT_USER" -m "$DEFAULT_USER"