workflows: add dependabot and deadnix

.github/CODEOWNERS

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+# Global owners
+*       @willbush

.github/dependabot.yaml

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+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      # Check for updates to GitHub Actions every week
+      interval: "weekly"

.github/workflows/deadnix.yaml

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+name: Dead code analysis
+on:
+  workflow_dispatch: # allows manual triggering
+  schedule:
+    - cron:
+        "0 0 1 * *" # It runs at midnight (0 hour 0 minute) of the 1st day
+        # of every month.
+
+jobs:
+  deadnix:
+    name: Deadnix
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: cachix/install-nix-action@v22
+      - uses: cachix/cachix-action@v12
+        with:
+          name: deadnix
+      - uses: astro/deadnix-action@main