Remove separate GH flow for demo test, run nix flake check on Mac as well

Author: vrom911

.github/workflows/ci-nix.yaml

@@ -188,7 +188,10 @@ jobs:
 
   nix-flake-check:
     name: "nix flake check"
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest]
     steps:
     - name: Reclaim the bytes
       uses: data-intuitive/reclaim-the-bytes@v2

.github/workflows/demo-test.yaml

@@ -185,6 +185,7 @@
               hydra-node
               pkgs.cardano-node
               pkgs.cardano-cli
+            ] ++ pkgs.lib.optionals pkgs.stdenv.isLinux [
               pkgs.dool
             ];
         };