Is your feature request related to a problem? Please describe.
I want direnv to use the shell produced by guix time-machine -C channels.scm -- shell -m manifest.scm when channels.scm is available, to allow the usage of guix fully reproducible environments with direnv.

Describe the solution you'd like
use guix should automatically invoke time-machine if a channels.scm is in the project root.

Describe alternatives you've considered
I am actually injecting this in .config/direnv/lib because I cannot find another way.