UpdateInfo: Start maintenancetool with low priority

Reflect that it is supposed to be a background activity.

Fixes: QTIFW-3024
Change-Id: I799552016c96e896992dd505cbfd86fa744c9502
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/plugins/updateinfo/updateinfoplugin.cpp

@@ -122,6 +122,7 @@ void UpdateInfoPlugin::startCheckForUpdates()
 
     const auto doSetup = [this](Process &process, const QStringList &args) {
         process.setCommand({d->m_maintenanceTool, args});
+        process.setLowPriority();
     };
     const auto doCleanup = [this] {
         d->m_taskTree.release()->deleteLater();