Use proxy server also for FTP .treeinfo download

We missed ftp prefix in the list of schemes.

Resolves: RHEL-27938

Author: jkonecny12

pyanaconda/modules/payloads/payload/dnf/tree_info.py

@@ -274,7 +274,8 @@ def _get_downloader(self, session, data):
                 proxy = ProxyString(proxy_url)
                 proxies = {
                     "http": proxy.url,
-                    "https": proxy.url
+                    "https": proxy.url,
+                    "ftp": proxy.url
                 }
             except ProxyStringError as e:
                 log.debug("Failed to parse the proxy '%s': %s", proxy_url, e)