Improve GitHub CI

[jakubnowicki]

First attempt: #475

Problems:
When using RSPM (Posit Package Manager), new package releases are available with a delay. renv understands that a package should be installed, but sometimes does throw an error that it is not available. The error is then thrown when the package is going to be used.

Example:
renv knows that it needs to install a particular version of xml2: https://github.com/jakubnowicki/RhinoTestCI/actions/runs/5475456323/job/14826039842#step:4:146
This version was not available (not yet on RSPM), but renv didn't throw an error.
As a result, an error occurred during the linter step, as xml2 is required by lintr: https://github.com/jakubnowicki/RhinoTestCI/actions/runs/5475456323/job/14826039842#step:4:146