Test the Nix packaging on CircleCI

.circleci/config.yml

@@ -25,6 +25,8 @@ workflows:
 
       - "slackware-14.2"
 
+      - "nixos-19.09"
+
       # Test against PyPy 2.7/7.1.1
       - "pypy2.7-7.1"
 
@@ -322,6 +324,21 @@ jobs:
       - store_artifacts: *STORE_OTHER_ARTIFACTS
       - run: *SUBMIT_COVERAGE
 
+  nixos-19.09:
+    docker:
+      # Run in a highly Nix-capable environment.
+      - image: "nixorg/nix:circleci"
+
+    environment:
+      NIX_PATH: "nixpkgs=https://github.com/NixOS/nixpkgs-channels/archive/nixos-19.09-small.tar.gz"
+
+    steps:
+      - "checkout"
+      - "run":
+          name: "Build and Test"
+          command: |
+            nix-build nix/
+
   # Generate up-to-date data for the dependency graph visualizer.
   build-porting-depgraph:
     # Get a system in which we can easily install Tahoe-LAFS and all its