Fix duplicated build token problem and added relevant spec

changelogs/unreleased/fix-import-export-build-token.yml

+---
+title: Fix Import/Export duplicated builds error
+merge_request: 
+author: 

lib/gitlab/import_export/relation_factory.rb

@@ -99,6 +99,8 @@ module Gitlab
       def generate_imported_object
         if BUILD_MODELS.include?(@relation_name) # call #trace= method after assigning the other attributes
           trace = @relation_hash.delete('trace')
+          @relation_hash.delete('token')
+
           imported_object do |object|
             object.trace = trace
             object.commit_id = nil

spec/lib/gitlab/import_export/project.json

@@ -6548,7 +6548,9 @@
             "url": null
           },
           "erased_by_id": null,
-          "erased_at": null
+          "erased_at": null,
+          "type": "Ci::Build",
+          "token": "abcd"
         },
         {
           "id": 72,

spec/lib/gitlab/import_export/project_tree_restorer_spec.rb

@@ -189,6 +189,14 @@ describe Gitlab::ImportExport::ProjectTreeRestorer, services: true do
           end
         end
       end
+
+      context 'existing builds' do
+        it 'do not throw errors' do
+          create(:ci_build, token: 'abcd')
+
+          expect(restored_project_json).to be true
+        end
+      end
     end
   end
 end