build: Fix typo that would lead to hitting the socket name length limit.

* tests/guix-register.sh: Remove redundant $new_store in $NIX_STATE_DIR,
  introduced in 689142cd ("guix-register: Add '--state-directory'
  parameter.")  Reported by Eric Bavier <ericbavier@gmail.com> and
  Alen Skondro <askondro@gmail.com>.