tech: add env var to disable cron scheduling

app/jobs/cron/cron_job.rb

@@ -4,7 +4,7 @@ class Cron::CronJob < ApplicationJob
 
   class << self
     def schedulable?
-      true
+      true && ENV['CRON_JOBS_DISABLED'].blank?
     end
 
     def schedule

config/env.example.optional

@@ -259,3 +259,6 @@ WEASYPRINT_URL="http://10.33.23.204:5000/pdf"
 
 # Use this env var customize the max number of deleted user per day
 EXPIRE_USER_DELETION_JOB_LIMIT=10000
+
+# write anything to disable cron jobs
+CRON_JOBS_DISABLED=""