fix: Rename CRON variable to BACKUP_CRON for clarity in backup schedule configuration

Author: AdrienPoupa

immich/backup.env.example

@@ -37,7 +37,7 @@ RESTIC_PASSWORD=
 #   0 30 3 * * * = 3:30 AM daily
 #   0 0 4 * * 0  = 4:00 AM every Sunday
 #   0 0 */12 * * * = Every 12 hours
-CRON=0 30 3 * * *
+BACKUP_CRON=0 30 3 * * *
 
 # Timezone for cron scheduling
 # Used to interpret the CRON schedule