[ hydra fix ] fix hostNixpkgs -> nixpkgs

hydra.nix

@@ -18,7 +18,7 @@
             "keepnr": 3,
             "inputs": {
               "argopkgs": { "type": "git", "value": "https://xgitlab.cels.anl.gov/argo/argopkgs.git", "emailresponsible": false },
-              "hostNixpkgs": { "type": "git", "value": "git://github.com/NixOS/nixpkgs.git release-18.03", "emailresponsible": false }
+              "nixpkgs": { "type": "git", "value": "git://github.com/NixOS/nixpkgs.git release-18.03", "emailresponsible": false }
             }
         }
     }