qa: Do not force overwrite of QT_QPA_PLATFORM on windows for gui tests

src/qt/test/test_main.cpp

@@ -61,7 +61,7 @@ int main(int argc, char *argv[])
     // platform ("xcb", "windows", or "cocoa") so tests can't unintentionally
     // interfere with any background GUIs and don't require extra resources.
     #if defined(WIN32)
-        _putenv_s("QT_QPA_PLATFORM", "minimal");
+        if (getenv("QT_QPA_PLATFORM") == nullptr) _putenv_s("QT_QPA_PLATFORM", "minimal");
     #else
         setenv("QT_QPA_PLATFORM", "minimal", /* overwrite */ 0);
     #endif