common: add dnsutils for ad-hoc DNS debugging

Same rationale as jq — useful when poking at the AdGuard / DNS path
during incidents, no package on the system currently provides dig/host.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

common.nix

@@ -98,6 +98,7 @@
       nvd
       nix-output-monitor
       jq
+      dnsutils
       busybox
   ];
 }