Hi,

I noticed `guix environment -C` lacks the two environment variable USER and
LOGNAME, which are part of %precious-variables (i.e. not cleared), but not
explicitly set when using a container. Some programs, in my case RStudio (see
https://github.com/leibniz-psychology/psychnotebook-app-rstudio/commit/3e15b9b173b8a9d2a7f55743ecba24e54b77ffad),
expect these to be set and can’t function properly without. Thus, the attached
patch exports them. This works with and without `-u`.

Cheers,
Lars