environment: '--link-profile' uses ~/.guix-profile for environment variables.

environment: '--link-profile' uses ~/.guix-profile for environment variables.

Before this patch, we had:

  $ guix environment -CP --ad-hoc coreutils
  [env]$ echo $PATH
  /gnu/store/…-profile/bin
  [env]$ echo $GUIX_ENVIRONMENT
  /gnu/store/…-profile

After this patch:

  $ guix environment -CP --ad-hoc coreutils
  [env]$ echo $PATH
  /home/ludo/.guix-profile/bin
  [env]$ echo $GUIX_ENVIRONMENT
  /home/ludo/.guix-profile

* guix/scripts/environment.scm (launch-environment/container): When
LINK-PROFILE? is true, pass ~/.guix-profile as the second argument to
'launch-environment'.
* tests/guix-environment-container.sh: Adjust test accordingly.
* doc/guix.texi (Invoking guix environment): Update accordingly.