Update sidecar_fallback.phpt

tests/ext/background-sender/sidecar_fallback.phpt

@@ -2,6 +2,7 @@
 Send telemetry about the sidecar being disabled
 --SKIPIF--
 <?php include __DIR__ . '/../includes/skipif_no_dev_env.inc'; ?>
+<?php die('skip: Sidecar is nowhere enabled by default yet.'); ?>
 <?php if (PHP_VERSION_ID < 80300) die('skip: Sidecar fallback exists only on PHP 8.3'); ?>
 <?php if (strncasecmp(PHP_OS, "WIN", 3) == 0) die('skip: There is no background sender on Windows'); ?>
 <?php if (getenv('USE_ZEND_ALLOC') === '0' && !getenv("SKIP_ASAN")) die('skip: valgrind reports sendmsg(msg.msg_control) points to uninitialised byte(s), but it is unproblematic and outside our control in rust code'); ?>