added build_muninn workflow

.github/workflows/build_muninn.yml

@@ -0,0 +1,18 @@
+name: build_muninn
+on:
+  workflow_dispatch:
+  pull_request:
+  push:
+    branches: [main]
+
+jobs:
+  build:
+    name: build_muninn
+    runs-on: ubuntu-22.04
+
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install Nix
+        uses: DeterminateSystems/nix-installer-action@v3
+      - name: Build default package
+        run: 'nix-shell -p nixos-rebuild --run "nixos-rebuild build --flake ./#muninn"'