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/searchhandler.cpp

@@ -73,6 +73,7 @@ SearchHandler::SearchHandler(const QString &pattern, const QString &category, co
 
     const QStringList params
     {
+        Utils::ForeignApps::PYTHON_ISOLATE_MODE_FLAG,
         (SearchPluginManager::engineLocation() / Path(u"nova2.py"_qs)).toString(),
         m_usedPlugins.join(u','),
         m_category