Tests of both R tutorial notebooks fail with nbclient.exceptions.CellTimeoutError, e.g. [here](https://github.com/iiasa/ixmp/actions/runs/3599025742/jobs/6069575193#step:14:1676). This occurs on all jobs that run on ubuntu-latest GitHub Actions runners, but not on Windows or macOS. Possible responses include: - Adjust the timeout time. - Temporarily or permanently skip or xfail the tests.