Set LOGNAME in `setup-r` (or `check-r-package`?)

Because if it is not set, then the detritus detection in `R CMD check` does not work properly. We can set it with something like:
```r
cat("LOGNAME=", Sys.info()[["user"]], "\n", sep = "", file = Sys.getenv("GITHUB_ENV"), append = TRUE)
```
