firmwre: make this clearer

Ideally, opnsense-update -M should replace this except
that -M is used to gain mirror access from other scripts
as well.  Need to think about it.

src/opnsense/scripts/firmware/product.php

@@ -32,7 +32,7 @@
 $ret = json_decode(@file_get_contents($metafile), true);
 if ($ret != null) {
     $ret['product_crypto'] = trim(shell_exec('opnsense-version -f'));
-    $ret['product_mirror'] = preg_replace('/\/[a-z0-9]{8}(-[a-z0-9]{4}){3}-[a-z0-9]{12}\//i', '/subscription-key/', trim(shell_exec('opnsense-update -M')));
+    $ret['product_mirror'] = preg_replace('/\/[a-z0-9]{8}(-[a-z0-9]{4}){3}-[a-z0-9]{12}\//i', '/${SUBSCRIPTION}/', trim(shell_exec('opnsense-update -M')));
     $ret['product_time'] = date('D M j H:i:s T Y', filemtime('/usr/local/opnsense/www/index.php'));
     $repos = explode("\n", trim(shell_exec('opnsense-verify -l')));
     sort($repos);