bug#35942: guix install: environment variable message is very confusing

[Robert Vollmert]

When installing a package that needs an environment variaible to be set for
use, `guix install` prints e.g.:

$ guix install python
...
The following environment variable definitions may be needed:
   export 
PYTHONPATH="/home/rob/.guix-profile/lib/python3.7/site-packages${PYTHONPATH:+:}$PYTHONPATH"

I understand that the variable will be set in a new shell, but not in the 
current shell.
But as it stands, the message serves to confuse even users that are somewhat 
experienced
with unix-like systems.

Suggestion to instead print:

Set the following environment variables to use <package> right away:
   export 
PYTHONPATH="/home/rob/.guix-profile/lib/python3.7/site-packages${PYTHONPATH:+:}$PYTHONPATH"