ci: add Dependabot Nix updates

Author: sith-traya

.github/dependabot.yml

@@ -4,4 +4,9 @@ updates:
     directory: "/"
     schedule:
       # Check for updates to GitHub Actions every week
-      interval: "weekly"
+      interval: "weekly"
+
+  - package-ecosystem: "nix"
+    directory: "/"
+    schedule:
+      interval: "weekly"