I'd like to change my Guix System to build packages in a different location (`/dev/shm/`) so that they are built on a tmpfs partition to hopefully speed up builds (defaults to `/tmp` which is on the `/` partition on Guix System). The Guix Manual only mentions that this may be done by setting the environment variable `TMPDIR`.

I was able to do this for a system that is configured as a foreign install by modifying the SystemD service file (Guix Mailing List), but I haven't been able to figure out how to modify my `/etc/config.scm` file to do this on Guix System.