bug#20255: 'search-paths' should respect both user and system profile.

[Ludovic Courtès]

Some progress has been made: fc2d233 allows search paths for multiple
profiles to be combined.

So I think I will eventually (‘guix-devel’ needs to be updated first)
change /etc/profile to do:

  eval `guix package -p /run/current-system/profile \
          -p $HOME/.guix-profile --search-paths`

That should solve the combined profile issue.

This operation takes ~400ms on my machine.  This would be a problem if
we had to do it every time a shell is started, but here we only need to
do it for log-in shells, which is rare enough.

WDYT?

Thanks,
Ludo’.