Reapply "Dump JSON directly to file instead of buffering"

This reverts commit 30e16ee.

Author: skipkayhil

ruby/lib/minitest/queue/build_status_reporter.rb

@@ -107,11 +107,11 @@ def progress
       end
 
       def write_failure_file(file)
-        File.write(file, error_reports.map(&:to_h).to_json)
+        JSON.dump(error_reports.map(&:to_h), File.open(file, 'w'))
       end
 
       def write_flaky_tests_file(file)
-        File.write(file, flaky_reports.to_json)
+        JSON.dump(flaky_reports, File.open(file, 'w'))
       end
 
       private

ruby/lib/minitest/queue/runner.rb

@@ -323,7 +323,7 @@ def display_warnings(build)
         warnings = build.pop_warnings.map do |type, attributes|
           attributes.merge(type: type)
         end.compact
-        File.write(queue_config.warnings_file, warnings.to_json)
+        JSON.dump(warnings, File.open(queue_config.warnings_file, 'w'))
       end
 
       def run_tests_in_fork(queue)