fix: add missing cron:set command

The code existed but somehow it wasn't exposed as a command.

Closes #6655

plugins/cron/src/commands/commands.go

@@ -20,7 +20,8 @@ Additional commands:`
 	helpContent = `
     cron:list <app> [--format json|stdout], List scheduled cron tasks for an app
     cron:report [<app>] [<flag>], Display report about an app
-    cron:run <app> <cron_id> [--detach], Run a cron task on the fly`
+    cron:run <app> <cron_id> [--detach], Run a cron task on the fly
+	cron:set [--global|<app>] <key> <value>, Set or clear a cron property for an app`
 )
 
 func main() {