Hi,

At the moment there doesn't seem to be any way to request ANSI stdout/err
from Guix if your output is not a tty.  However when capturing Guix
output for display inside a front-end for running jobs across many
servers (eg Jenkins, Rundeck, etc) - these tools are often capable of
rendering ANSI output captured from console tools like Guix.  Other
tools (eg py-test, flake8) often allow the user to explicitly force
color output even when a tty is not detected.  The attached patch is a
trivial attempt at make such an option available in Guix too.

I've done this in the least disruptive way possible, which is to
introduce an new env var such that users of the NO_COLOR env var will
get continuity of functionality.

Cheers,
Phil