ci(dependabot): add dependabot for nix flake updates

See https://github.blog/changelog/2026-04-07-dependabot-version-updates-now-support-the-nix-ecosystem/

Author: JamieMagee

.github/dependabot.yml

@@ -21,3 +21,8 @@ updates:
     directory: "/derive"
     schedule:
       interval: "monthly"
+
+  - package-ecosystem: "nix"
+    directory: "/"
+    schedule:
+      interval: "monthly"