Fix tar invocation on FreeBSD
Fix tar invocation on FreeBSD

tar(1) on FreeBSD does not use standard output or input when the -f flag
is not provided. Instead, it defaults to /dev/sa0 on FreeBSD.

Make this tar invocation a bit more robust and explicitly tell tar(1) to
use standard output.

This is one of the issues discovered while porting Nix to FreeBSD. It has
been tested and committed locally to FreeBSD ports:
https://svnweb.freebsd.org/ports/head/sysutils/nix/Makefile?revision=550026&view=markup#l108
rycee/nix,NixOS/nix,NixOS/nix,NixOS/nix,NixOS/nix,rycee/nix,rycee/nix,NixOS/nix,rycee/nix,rycee/nix