workflows: addition of nix cache workflow

.github/workflows/nix-cache.yaml

@@ -0,0 +1,19 @@
+name: "Populate cachix cache"
+on:
+  workflow_dispatch:
+  push:
+    branches: [ master ]
+    paths: [ 'src/**.rs', 'Cargo.toml', 'Cargo.lock', 'nix/package.nix' ]
+jobs:
+  populate-cache:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - uses: cachix/install-nix-action@v25
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
+    - uses: cachix/cachix-action@v14
+      with:
+        name: nyx
+        authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+    - run: nix build