Buildkite: add workaround for input-output-hk/iohk-ops#549

Author: rvl

.buildkite/pipeline.yml

@@ -1,9 +1,9 @@
 env:
-  NIX_PATH: "nixpkgs=channel:nixos-19.03"
+  NIX_PATH: "nixpkgs=channel:nixos-18.09"
   NIX_BUILD_ARGS: "--cores 0 --max-jobs 2"
 
 steps:
   - label: 'Run tests'
-    command: ./test/tests.sh
+    command: "NIX_PATH=nixpkgs=channel:nixos-18.09 ./test/tests.sh"
     agents:
       system: x86_64-linux