ci: Update dependabot to look for dependencies in nested folder

- Bump cachix/install-nix-action from 21 to 22

- Add actions/setup-nix to dependabot dependencies

.github/actions/setup-nix/action.yaml

@@ -11,7 +11,7 @@ inputs:
 runs:
   using: composite
   steps:
-    - uses: cachix/install-nix-action@v18
+    - uses: cachix/install-nix-action@v22
       with:
         install_url: https://releases.nixos.org/nix/nix-2.13.3/install
     - uses: cachix/cachix-action@v12

.github/dependabot.yml

@@ -1,6 +1,11 @@
 version: 2
 updates:
-- package-ecosystem: github-actions
-  directory: /
-  schedule:
-    interval: weekly
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly
+
+  - package-ecosystem: github-actions
+    directory: /.github/actions/setup-nix
+    schedule:
+      interval: weekly