[frontend] Refactor BuildControlle#result_lastsuccess

Use Array#map instead of using a temporary variable to collect arch
results.

src/api/app/controllers/build_controller.rb

@@ -165,15 +165,12 @@ def result_lastsuccess
     bs = PackageBuildStatus.new(pkg).result(target_project: tprj, srcmd5: params[:srcmd5], multibuild_pkg: multibuild_package)
     @result = []
     bs.each do |repo, status|
-      archs = []
-      status.each do |arch, archstat|
-        oneline = [arch, archstat[:result]]
+      archs = status.map do |arch, archstat|
         if archstat[:missing].blank?
-          oneline << nil
+          [arch, archstat[:result], nil]
         else
-          oneline << archstat[:missing].join(',')
+          [arch, archstat[:result], archstat[:missing].join(',')]
         end
-        archs << oneline
       end
       @result << [repo, archs]
     end