Prefere conda to install R packages

[pierrepo]

See this template repo: https://github.com/binder-examples/r-conda

R packages not (yet) available in conda could be installed manually in a postBuild script.

[pierrepo]

See also https://github.com/ngs-docs/2020-ggg-201b-rnaseq/blob/master/binder/environment.yml