Make channels?part=membership deterministic

As requested by someone on [Discord](https://discord.com/channels/@me/1127939666792951859/1235245766789042246).

Verified with 12 requests in a row thanks to:

```py
import requests
from tqdm import tqdm

url = 'http://localhost/YouTube-operational-API/channels'
params = {
    'part': 'membership',
    'id': 'UCADXSO2msINXrTXAHxKoUgg',
}
progressBar = tqdm()
while True:
    response = requests.get(url, params)
    if response.status_code == 500 or response.json()['items'][0]['isMembershipEnabled'] != True:
        break
    progressBar.update(1)
```

channels.php

@@ -382,7 +382,7 @@ function getItem($id, $order, $continuationToken)
 
         if ($options['membership']) {
             $result = getJSONFromHTML("https://www.youtube.com/channel/$id");
-            $item['isMembershipEnabled'] = array_key_exists('sponsorButton', $result['header']['c4TabbedHeaderRenderer']);
+            $item['isMembershipEnabled'] = doesPathExist($result, 'header/c4TabbedHeaderRenderer/sponsorButton') || getValue($result, 'header/pageHeaderRenderer/content/pageHeaderViewModel/actions/flexibleActionsViewModel/actionsRows/0/actions/1/buttonViewModel/targetId', $defaultValue = null) === 'sponsorships-button';
         }
 
         if ($options['playlists']) {