Automatic RStudio upgrades

[jemrobinson]

:white_check_mark: Checklist

• [x] I have searched open and closed issues for duplicates.
• [x] This is a request for a new feature in the Data Safe Haven or an upgrade to an existing feature.
• [x] The feature is still missing in the latest version.
• [x] I have read through the documentation.
• [x] This isn't an open-ended question (open a discussion if it is).

:strawberry: Suggested change

Currently we install a hard-coded RStudio version. We should enable automatic upgrades.

:steam_locomotive: How could this be done?

1. Run our own (internal?) PPA which downloads deb files automatically
2. Point to an external PPA or repo for this file
3. Update the metadata for the file we're interested in (probably not this one)