sc: XLSX - Use correct index in svExternalName

This is related to f85d860ccb, for
`svExternalName` use the index calculated during export.

bug-document: forum-mso-de-38936.xls

Change-Id: I4a75a2432ab9bd6f46d36c46fe985bbc33252154
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/197917
Reviewed-by: Michael Stahl <michael.stahl@collabora.com>
Tested-by: Jenkins CollaboraOffice <jenkinscollaboraoffice@gmail.com>
(cherry picked from commit b14a56012663adfe529b32fb78033a98af1b3847)
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/199424
Tested-by: Jenkins
Reviewed-by: Karthik Godha <karthik.godha@collabora.com>

sc/source/core/tool/compiler.cxx

@@ -5422,7 +5422,8 @@ void ScCompiler::CreateStringFromExternal( OUStringBuffer& rBuffer, const Formul
     switch (t->GetType())
     {
         case svExternalName:
-            rBuffer.append(pConv->makeExternalNameStr( nFileId, *pFileName, t->GetString().getString()));
+            rBuffer.append(
+                pConv->makeExternalNameStr(nUsedFileId, *pFileName, t->GetString().getString()));
         break;
         case svExternalSingleRef:
             pConv->makeExternalRefStr(rDoc.GetSheetLimits(),