Update nix setup, add flake validation job

.github/workflows/ci-pipeline.yml

@@ -93,6 +93,31 @@ jobs:
           name: release_notes_bins
           path: ./release_notes_bins/
 
+  validate-nix-flake:
+    name: Validate Nix Flake
+    needs: start-gate
+    runs-on: ubuntu-latest
+    steps:
+      - name: Setup Nix
+        uses: cachix/install-nix-action@v31
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+
+      - name: Checkout (Branch)
+        uses: actions/checkout@v4
+        if: github.event_name == 'push' || github.event_name == 'schedule'
+
+      - name: Checkout (PR Merge)
+        uses: actions/checkout@v4
+        if: github.event_name != 'push' && github.event_name != 'schedule'
+        with:
+          ref: "refs/pull/${{ inputs.pull_request_number }}/merge"
+
+      - name: Check Flake
+        run: |
+          cd build/package/nix
+          nix flake check
+
   code-scanning:
     name: Run CodeQL
     needs: start-gate
@@ -1695,6 +1720,7 @@ jobs:
         check-winget-pr-template,
         efcore-version-match,
         code-scanning,
+        validate-nix-flake,
       ]
     runs-on: ubuntu-latest
     steps:

.github/workflows/nix-deployment.yml

@@ -15,7 +15,7 @@ jobs:
           sudo apt-get install -y xmlstarlet
 
       - name: Setup Nix
-        uses: cachix/install-nix-action@v30
+        uses: cachix/install-nix-action@v31
         with:
           nix_path: nixpkgs=channel:nixos-unstable