[fax] replace bg_system with os.execute (#6462)

to support systems that have the system commands disabled

app/scripts/resources/scripts/app/fax/resources/scripts/hangup_rx.lua

@@ -233,7 +233,8 @@
 		cmd = cmd .. "fax_prefix=false ";
 	end
 	freeswitch.consoleLog("notice", "[fax] command: " .. cmd .. "\n");
-	result = api:executeString("bg_system "..cmd);
+	os.execute(cmd);
+	
 
 --add to fax logs
 	sql = "insert into v_fax_logs ";