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

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

/nix/store/sdm3cgm0yg9mzizfpp0w75y4n6n3cgn6-gitlab-10.8.0/share/gitlab/bin/parallel-rsync-repos: interpreter directive changed from "/usr/bin/env bash" to "/nix/store/fqm2x6kiay1q4vg7pqp4wp17bdijlyc3-bash-4.4-p23/bin/bash"
/nix/store/sdm3cgm0yg9mzizfpp0w75y4n6n3cgn6-gitlab-10.8.0/share/gitlab/bin/mail_room: interpreter directive changed from "/bin/sh" to "/nix/store/fqm2x6kiay1q4vg7pqp4wp17bdijlyc3-bash-4.4-p23/bin/sh"
/nix/store/sdm3cgm0yg9mzizfpp0w75y4n6n3cgn6-gitlab-10.8.0/share/gitlab/bin/daemon_with_pidfile: interpreter directive changed from "/usr/bin/env ruby" to "/nix/store/yz441arw05zkax4gnprkyrklikyya3p0-wrapped-ruby-gitlab-env-10.8.0/bin/ruby"
/nix/store/sdm3cgm0yg9mzizfpp0w75y4n6n3cgn6-gitlab-10.8.0/share/gitlab/bin/check: interpreter directive changed from "/bin/sh" to "/nix/store/fqm2x6kiay1q4vg7pqp4wp17bdijlyc3-bash-4.4-p23/bin/sh"
/nix/store/sdm3cgm0yg9mzizfpp0w75y4n6n3cgn6-gitlab-10.8.0/share/gitlab/bin/changelog: interpreter directive changed from "/usr/bin/env ruby" to "/nix/store/yz441arw05zkax4gnprkyrklikyya3p0-wrapped-ruby-gitlab-env-10.8.0/bin/ruby"
/nix/store/sdm3cgm0yg9mzizfpp0w75y4n6n3cgn6-gitlab-10.8.0/share/gitlab/bin/bundle: interpreter directive changed from "/usr/bin/env ruby" to "/nix/store/yz441arw05zkax4gnprkyrklikyya3p0-wrapped-ruby-gitlab-env-10.8.0/bin/ruby"
/nix/store/sdm3cgm0yg9mzizfpp0w75y4n6n3cgn6-gitlab-10.8.0/share/gitlab/bin/background_jobs: interpreter directive changed from "/bin/sh" to "/nix/store/fqm2x6kiay1q4vg7pqp4wp17bdijlyc3-bash-4.4-p23/bin/sh"
/nix/store/sdm3cgm0yg9mzizfpp0w75y4n6n3cgn6-gitlab-10.8.0/share/gitlab/Rakefile: interpreter directive changed from "/usr/bin/env rake" to "/nix/store/rf8hwvs1f6l354j1s3fc1f91jmysd7xz-gitlab-env-10.8.0/bin/rake"
checking for references to /build in /nix/store/sdm3cgm0yg9mzizfpp0w75y4n6n3cgn6-gitlab-10.8.0...
/nix/store/sdm3cgm0yg9mzizfpp0w75y4n6n3cgn6-gitlab-10.8.0

copying path '/nix/store/zf6whqs8cj6ghwn0klf72xxq2dlz77ii-ruby2.5.1-xpath-2.1.0' from 'https://cache.nixos.org'...
copying path '/nix/store/qa6n141sxmnr6hyd9fvlzpjwnja6z2mm-ruby2.5.1-fog-aliyun-0.2.0' from 'https://cache.nixos.org'...
copying path '/nix/store/8v9ndbv1jq731d6d1pkhhl8xg145givz-ruby2.5.1-capybara-2.15.1' from 'https://cache.nixos.org'...
copying path '/nix/store/4wagrlawp1b5q39xprbg2fp81c1hkvib-ruby2.5.1-license_finder-3.1.1' from 'https://cache.nixos.org'...
copying path '/nix/store/kbrkj1di4i264qszb1d99gh1j3fiqyip-ruby2.5.1-capybara-screenshot-1.0.14' from 'https://cache.nixos.org'...
copying path '/nix/store/n4h5bd53gbnrkinhzg8whmjs8i8zgvsz-ruby2.5.1-spinach-rails-0.2.1' from 'https://cache.nixos.org'...
copying path '/nix/store/3h554sq9c81lbqjxpscbvbd12aiqk4f9-gitlab-env-10.8.0' from 'https://cache.nixos.org'...
copying path '/nix/store/ljd6z0mrdm6xblq8yrw70jrn4c6w7v9w-wrapped-ruby-gitlab-env-10.8.0' from 'https://cache.nixos.org'...
copying path '/nix/store/y0lzfq5j0yn92czkqliigfqj2f7asd67-gitlab-10.8.0' from 'https://cache.nixos.org'...
/nix/store/y0lzfq5j0yn92czkqliigfqj2f7asd67-gitlab-10.8.0