nix: handle ref/rev correctly for channel fetch

nix/default.nix

@@ -10,7 +10,8 @@
         url = "https://github.com/NixOS/nixpkgs-channels/";
         # Commit hash for nixos-unstable as of 2018-09-12
         # `git ls-remote https://github.com/nixos/nixpkgs-channels nixos-unstable`
-        rev = (builtins.getAttr channel channelVersions);
+        ref = (builtins.getAttr channel channelVersions)."ref";
+        rev = (builtins.getAttr channel channelVersions)."rev";
         name = "nixpkgs-channels-${channel}";
       };
       nix_path = builtins.foldl' (sum: elem: sum +":" + builtins.concatStringsSep "=" elem) "" [

nix/os/devices/steveej-laptop/versions.nix

@@ -0,0 +1,10 @@
+{
+  channelsNixosStable = {
+    ref = "nixos-18.09";
+    rev = "7e88992a8c7b2de0bcb89182d8686b27bd93e46a";
+  };
+  channelsNixosUnstable = {
+    ref = "nixos-unstable";
+    rev = "44b02b52ea6a49674f124f50009299f192ed78bb";
+  };
+}

nix/os/devices/steveej-t480s-work/versions.nix

@@ -1,4 +1,10 @@
 {
-  channelsNixosStable = "7e88992a8c7b2de0bcb89182d8686b27bd93e46a";
-  channelsNixosUnstable = "44b02b52ea6a49674f124f50009299f192ed78bb";
+  channelsNixosStable = {
+    ref = "nixos-18.09";
+    rev = "7e88992a8c7b2de0bcb89182d8686b27bd93e46a";
+  };
+  channelsNixosUnstable = {
+    ref = "nixos-unstable";
+    rev = "44b02b52ea6a49674f124f50009299f192ed78bb";
+  };
 }

nix/variables/versions.nix

@@ -1,4 +1,10 @@
 {
-  channelsNixosStable = "7e88992a8c7b2de0bcb89182d8686b27bd93e46a";
-  channelsNixosUnstable = "44b02b52ea6a49674f124f50009299f192ed78bb";
+  channelsNixosStable = {
+    ref = "nixos-18.09";
+    rev = "7e88992a8c7b2de0bcb89182d8686b27bd93e46a";
+  };
+  channelsNixosUnstable = {
+    ref = "nixos-unstable";
+    rev = "44b02b52ea6a49674f124f50009299f192ed78bb";
+  };
 }