Nix installation now requires following redirects using `curl -L`. This
is currently represented on the [Nix download page][] but not in the
manual. This change updates the manual to reflect this.
Using `curl` without the `-L` flag results in an empty body, making
installation a no-op.
[Nix download page]: https://nixos.org/download.html
(cherry picked from commit 9069759767)
Signed-off-by: Domen Kožar <domen@dev.si>
