Use python isolate mode

This (more or less) avoids user's environment variables tampering the
search process.
And also remove usages of `eval()` and `exec()`.

PR #18995.

src/base/search/searchdownloadhandler.cpp

@@ -46,6 +46,7 @@ SearchDownloadHandler::SearchDownloadHandler(const QString &siteUrl, const QStri
             , this, &SearchDownloadHandler::downloadProcessFinished);
     const QStringList params
     {
+        Utils::ForeignApps::PYTHON_ISOLATE_MODE_FLAG,
         (SearchPluginManager::engineLocation() / Path(u"nova2dl.py"_qs)).toString(),
         siteUrl,
         url