ci: add Nix cache workflow

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I08a763b36c1794ea33bfb7b10467328f6a6a6964

Author: NotAShelf

.github/workflows/nix-cache.yml

@@ -0,0 +1,30 @@
+name: Build and Cache with Nix
+
+on:
+  workflow_dispatch:
+  push:
+    branches: [ "main" ]
+    paths: [ 'watt/**.rs', 'xtask/*', 'build.rs',  'Cargo.toml', 'Cargo.lock', 'nix/package.nix', 'flake.nix', 'flake.lock' ]
+
+permissions:
+  contents: read
+
+jobs:
+  populate-cache:
+    runs-on: ubuntu-latest
+    steps:
+      - name: "Checkout"
+        uses: actions/checkout@v6
+
+      - uses: cachix/install-nix-action@v31
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+
+      - uses: cachix/cachix-action@v16
+        with:
+          name: watt
+          authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+
+      - name: "Build with Nix"
+        run: nix build
+