fix: replace instanceof check (#6284)

error instanceof EditorialWorkflowError was returning false
for a custom backend that inherited from GitGatewayBackend

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

packages/netlify-cms-core/src/backend.ts

@@ -5,10 +5,10 @@ import {
   localForage,
   Cursor,
   CURSOR_COMPATIBILITY_SYMBOL,
-  EditorialWorkflowError,
   getPathDepth,
   blobToFileObj,
   asyncLock,
+  EDITORIAL_WORKFLOW_ERROR,
 } from 'netlify-cms-lib-util';
 import { basename, join, extname, dirname } from 'path';
 import { stringTemplate } from 'netlify-cms-lib-widgets';
@@ -417,7 +417,7 @@ export class Backend {
       (await this.implementation
         .unpublishedEntry({ collection: collection.get('name'), slug })
         .catch(error => {
-          if (error instanceof EditorialWorkflowError && error.notUnderEditorialWorkflow) {
+          if (error.name === EDITORIAL_WORKFLOW_ERROR && error.notUnderEditorialWorkflow) {
             return Promise.resolve(false);
           }
           return Promise.reject(error);