bug fix: preventing updating submission status with a non-integer value

[yutanakamura-tky]

The status field in submission_info is supposed to be an integer defined by the SubmissionStatus class, but there was one place in the script where the status was still being overwritten with the string "done". This occasionally caused an error where the leaderboard would not display correctly. Therefore, this pull request fixes the issue by replacing the string assignment with SubmissionStatus.SUCCESS.value, thus preventing the bug where status could become a non-integer value.