fixup! Add Buildkite pipeline for updating hackage.nix and stackage.nix

Author: rvl

.buildkite/updates.yml

@@ -1,20 +1,16 @@
 steps:
   - label: 'Update hackage.nix'
     command:
-      - nix-build -A maintainer-scripts.update-hackage -o update-hackage.sh
+      - nix-build -I nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/nixos-18.09.tar.gz -A maintainer-scripts.update-hackage -o update-hackage.sh
       - echo "+++ Updating hackage.nix"
       - ./update-hackage.sh
-    env:
-      NIX_PATH: "nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/nixos-18.09.tar.gz"
     agents:
       system: x86_64-linux
 
   - label: 'Update stackage.nix'
     command:
-      - nix-build -A maintainer-scripts.update-stackage -o update-stackage.sh
+      - nix-build -I nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/nixos-18.09.tar.gz -A maintainer-scripts.update-stackage -o update-stackage.sh
       - echo "+++ Updating stackage.nix"
       - ./update-stackage.sh
-    env:
-      NIX_PATH: "nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/nixos-18.09.tar.gz"
     agents:
       system: x86_64-linux