Hi everyone,

the attched patch allows starting a `guix environment` from an existing
profile. Our use-case is this: We have profiles, created by

	guix package -p /path -i …

because we need to parse desktop files in /path/share/applications. Also
this does the heavy-lifting (substitutes/building/grafting) upfront once
and not when starting an environment. Then we start an application from
this profile using

	guix environment -C […] -p /path -- jupyterlab […]

This is much faster (10x) for large profiles (i.e. JupyterLab) than

	guix environment -C […] -m manifest.scm -- jupyterlab […]

and makes sure the environment is exactly the same as

	source /path/etc/profile

I think there have been many discussions about how `guix environment`
should work and this is certainly not the silver bullet, but it’s a huge
improvement for us with very limited code changes required.

Cheers,
Lars