common: install jq globally

Useful for ad-hoc shell scripts (e.g. downloads-cleanup.sh) — already a
build-time dep of arr-interconnect, just wasn't on the user PATH.

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

common.nix

@@ -97,6 +97,7 @@
       localsend
       nvd
       nix-output-monitor
+      jq
       busybox
   ];
 }