Do not override QT_QPA_PLATFORM env variables

The QGuiApplication documentation explicitly says that QT_QPA_PLATFORM
can be used to specify the QPA for every QGuiApplication. Unconditionally
overriding this value within the application is a unexpected behavior.

Task-number: QTBUG-54789
Change-Id: Ia7a61a0f2921ef8ec27fe630e8d307d20ba1eb37
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

tools/qmlplugindump/main.cpp

@@ -977,7 +977,7 @@ int main(int argc, char *argv[])
         }
     }
 
-    if (!requireWindowManager)
+    if (!requireWindowManager && qEnvironmentVariableIsEmpty("QT_QPA_PLATFORM"))
         qputenv("QT_QPA_PLATFORM", QByteArrayLiteral("minimal"));
     else
         QCoreApplication::setAttribute(Qt::AA_ShareOpenGLContexts, true);