Set NIX_PATH in CI

Author: thomasjm

.github/workflows/ci.yml

@@ -40,4 +40,5 @@ jobs:
     - name: Run nix-build
       run: |
         cd test_depots/${{matrix.test_depot}}
+        export NIX_PATH="nixpkgs=$(nix run .#nixpkgsPath)"
         nix-build .

flake.nix

@@ -24,6 +24,8 @@
                --set PATH ${lib.makeBinPath [nix python3 julia-stable-bin]} \
                --set NIX_PATH nixpkgs=${pkgs.path}
             '';
+
+            nixpkgsPath = pkgs.writeShellScriptBin "nixpkgsPath.sh" "echo -n ${pkgs.path}";
           };
         });
 }