ci: add nix workflows

Signed-off-by: seth <getchoo@tuta.io>

.github/workflows/nix.yml

@@ -0,0 +1,44 @@
+name: Nix
+
+on:
+  push:
+    branches: ['main']
+  pull_request:
+  workflow_dispatch:
+
+jobs:
+  build:
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest]
+
+    runs-on: ${{ matrix.os }}
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Install Nix
+        uses: DeterminateSystems/nix-installer-action@v8
+
+      - name: Setup Nix cache
+        uses: DeterminateSystems/magic-nix-cache-action@v2
+
+      - name: Build refraction
+        run: nix build -L --fallback
+
+  check:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Install Nix
+        uses: DeterminateSystems/nix-installer-action@v8
+
+      - name: Setup Nix cache
+        uses: DeterminateSystems/magic-nix-cache-action@v2
+
+      - name: Run checks
+        run: nix flake check -L --show-trace

.github/workflows/update-flake.yml

@@ -0,0 +1,28 @@
+name: Update flake.lock
+
+on:
+  schedule:
+    # run every saturday
+    - cron: '0 0 * * 6'
+  workflow_dispatch:
+
+jobs:
+  update:
+    runs-on: ubuntu-latest
+
+    permissions:
+      contents: write
+      pull-requests: write
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install Nix
+        uses: nixbuild/nix-quick-install-action@v26
+
+      - name: Update and create PR
+        uses: DeterminateSystems/update-flake-lock@v20
+        with:
+          commit-msg: 'flake: update inputs'
+          pr-title: 'flake: update inputs'
+          token: ${{ github.token }}