Additionally, propagate the GUIX_TERRAFORM_PLUGIN_DIR variable so that only
guix installed terraform providers can be used. This has the effect of
disabling automatic fetching of terraform providers, which may be inconvenient
to those who are not expecting this behavior. On Nix, this is resolved by not
using a shim shell script that sets this variable when no terraform providers
are installed. This approach doesn't work very well on guix, so in the future
separate packages which inherit from the existing terraform packages but do
not set this environment variable may be provided.
