chore(ci/nix): port nix related github actions

.github/workflows/cachix.yml

@@ -0,0 +1,26 @@
+name: cachix
+
+on:
+  push:
+    branches:
+      - v5
+  workflow_run:
+    workflows: ["update flake"]
+    types:
+      - completed
+    branches:
+      - v5
+  workflow_dispatch:
+
+jobs:
+  cache:
+    runs-on: ubuntu-latest
+    if: ${{ github.repository == 'noctalia-dev/noctalia-shell' }}
+    steps:
+      - uses: actions/checkout@v6
+      - uses: cachix/install-nix-action@v31
+      - uses: cachix/cachix-action@v17
+        with:
+          name: "${{ secrets.CACHIX_CACHE_NAME }}"
+          authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
+      - run: nix build .

.github/workflows/update-flake.yml

@@ -0,0 +1,20 @@
+name: update flake
+
+on:
+  schedule:
+    - cron: "0 0 * * 0"
+  workflow_dispatch:
+
+jobs:
+  update:
+    runs-on: ubuntu-latest
+    if: ${{ github.repository == 'noctalia-dev/noctalia-shell' }}
+    permissions:
+      contents: write
+    steps:
+      - uses: actions/checkout@v6
+      - uses: cachix/install-nix-action@v31
+      - run: nix flake update
+      - uses: stefanzweifel/git-auto-commit-action@v7
+        with:
+          commit_message: "chore(flake): update flake.lock"