Set `lastUpdate` to zero when clearing packages

Since reloading packages is now avoided clearing packages should not itself
count as update as it would prevent loading packages again.

libpkg/data/database.cpp

@@ -111,7 +111,7 @@ void Database::resetConfiguration()
 
 void Database::clearPackages()
 {
-    lastUpdate = CppUtilities::DateTime::gmtNow();
+    lastUpdate = DateTime();
     if (m_storage) {
         m_storage->packageCache.clear(*m_storage);
     }