guix package: Make custom profiles actual indirect roots.

guix package: Make custom profiles actual indirect roots.

Before that, any profile generation built when '-p' is used would
effectively become a permanent GC root because the symlink in
/var/guix/gcroots/auto would point directly to /gnu/store/...-profile.

* guix/scripts/package.scm (maybe-register-gc-root): Rename to...
  (register-gc-root): ... this.  Remove conditional, and replace call to
  'canonicalize-path' with (string-append (getcwd) "/" ...).
  (guix-package): Call 'register-gc-root' only if PROFILE is different
  from %CURRENT-PROFILE.
* tests/guix-package.sh: Add test case.