ci: fix R 3.6 incompatibility because of change in the default value of StringAsFactor in R 4.0

Refs #25
2 jobs for 25-set-griwrm-rownames-with-id in 3 minutes and 42 seconds (queued for 9 seconds)
Status Name Job ID Coverage
  Checks
passed check_as_cran #141253
docker

00:03:40

passed check_not_cran #141252
docker

00:03:42