a) For `nixos-rebuild` you can set
  { nixpkgs.config.allowUnfree = true; }
in configuration.nix to override this.

b) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
  { allowUnfree = true; }
to ~/.config/nixpkgs/config.nix.

unpacking source archive /nix/store/17m02fq48gkyhrshrwm2c3lip9lk0pdr-zookeeper-3.4.12.tar.gz
source root is zookeeper-3.4.12
setting SOURCE_DATE_EPOCH to timestamp 1522125433 of file zookeeper-3.4.12/dist-maven/zookeeper-3.4.12-tests.jar.sha1.asc
installing
patching script interpreter paths in /nix/store/15qv6af3l68jvfs0292bk9dr58g0q4ls-zookeeper-3.4.12/bin
/nix/store/15qv6af3l68jvfs0292bk9dr58g0q4ls-zookeeper-3.4.12/bin/zkCli.sh: interpreter directive changed from "/usr/bin/env bash" to "/nix/store/8zkg9ac4s4alzyf4a8kfrig1j73z66dw-bash-4.4-p23/bin/bash"
/nix/store/15qv6af3l68jvfs0292bk9dr58g0q4ls-zookeeper-3.4.12/bin/zkCleanup.sh: interpreter directive changed from "/usr/bin/env bash" to "/nix/store/8zkg9ac4s4alzyf4a8kfrig1j73z66dw-bash-4.4-p23/bin/bash"
/nix/store/15qv6af3l68jvfs0292bk9dr58g0q4ls-zookeeper-3.4.12/bin/zkEnv.sh: interpreter directive changed from "/usr/bin/env bash" to "/nix/store/8zkg9ac4s4alzyf4a8kfrig1j73z66dw-bash-4.4-p23/bin/bash"
/nix/store/15qv6af3l68jvfs0292bk9dr58g0q4ls-zookeeper-3.4.12/bin/zkServer.sh: interpreter directive changed from "/usr/bin/env bash" to "/nix/store/8zkg9ac4s4alzyf4a8kfrig1j73z66dw-bash-4.4-p23/bin/bash"
/nix/store/15qv6af3l68jvfs0292bk9dr58g0q4ls-zookeeper-3.4.12