Use block-form of File.open and let Ruby do the cleanup

Library/Homebrew/formula.rb

@@ -572,7 +572,7 @@ class Formula
       end
       wr.close
 
-      f = File.open(logfn, 'w')
+      File.open(logfn, 'w') do |f|
         f.write(rd.read) until rd.eof?
 
         Process.wait
@@ -586,10 +586,11 @@ class Formula
           raise ErrorDuringExecution
         end
       end
+    end
   rescue ErrorDuringExecution
     raise BuildError.new(self, cmd, args, $?)
   ensure
-    [rd, f].each { |io| io.close if io and not io.closed? }
+    rd.close if rd and not rd.closed?
     ENV.update(removed_ENV_variables) if removed_ENV_variables
   end