[GraphEditor] Use attribute's evaluated value for "Open File"

Otherwise, input attributes with variables (either environment ones
or local ones) cannot be opened through "Open File", even though "Open
Containing Folder" works fine.

meshroom/ui/qml/GraphEditor/AttributeItemDelegate.qml

@@ -152,7 +152,7 @@ RowLayout {
                             visible: paramMenu.isFilepath
                             height: visible ? implicitHeight : 0
                             text: "Open File"
-                            onClicked: Qt.openUrlExternally(Filepath.stringToUrl(attribute.value))
+                            onClicked: Qt.openUrlExternally(Filepath.stringToUrl(attribute.evalValue))
                         }
                     }