Fix force-choosing gtk3 backend.

Extra parameter got lost in refactoring, if we set envvar, we *do*
want to try that backend.

Change-Id: Ib530a3a6b3e6ee33aab729ce7cd1f93e209d4513

vcl/unx/generic/plugadapt/salplug.cxx

@@ -214,7 +214,7 @@ SalInstance *CreateSalInstance()
         aUsePlugin = "svp";
 
     if( !aUsePlugin.isEmpty() )
-        pInst = tryInstance( aUsePlugin );
+        pInst = tryInstance( aUsePlugin, true );
 
     if( ! pInst )
         pInst = autodetect_plugin();