Idea: when in a guix environment, have guix install packages by default to that environment

[pkill9]

darth-cheney asked in IRC if from inside a guix environment you would
install the packages instead of pip/node, and maybe having guix install
packages to current guix environment would be a good idea.

Possible issue would be how to change the current guix environment to
the new profile, without creating a nest of environments for every
`guix install` or `guix package -i`.