fix(core): Allow ext. auth to find customer on any channel

Relates to #961

packages/core/src/service/helpers/external-authentication/external-authentication.service.ts

@@ -36,17 +36,26 @@ export class ExternalAuthenticationService {
      * @description
      * Looks up a User based on their identifier from an external authentication
      * provider, ensuring this User is associated with a Customer account.
+     *
+     * By default, only customers in the currently-active Channel will be checked.
+     * By passing `false` as the `checkCurrentChannelOnly` argument, _all_ channels
+     * will be checked.
      */
     async findCustomerUser(
         ctx: RequestContext,
         strategy: string,
         externalIdentifier: string,
+        checkCurrentChannelOnly = true,
     ): Promise<User | undefined> {
         const user = await this.findUser(ctx, strategy, externalIdentifier);
 
         if (user) {
             // Ensure this User is associated with a Customer
-            const customer = await this.customerService.findOneByUserId(ctx, user.id);
+            const customer = await this.customerService.findOneByUserId(
+                ctx,
+                user.id,
+                checkCurrentChannelOnly,
+            );
             if (customer) {
                 return user;
             }