Add rig repos ...

[gaborcsardi]

To query and configure repositories. In particular have a config option (do we need rig config?) to whether we should set the repos in RStudio (and Positron) or not.

Instead of rig config we can also write the config into the profile.

We should be able to configure different repos for different R versions:

• R-devel does not have PPM binaries.
• Older macOS and Windows binaries are on cran-archive.

By default all these should be configured automatically. (Observing the --without-cran-mirror and --without ppm options of rig add.)