[CI Problem] run-comment-bot fails when it can't get the PR number

[areusch]

See https://github.com/apache/tvm/actions/runs/3097181204/jobs/5013784930.

Traceback (most recent call last):
  File "ci/scripts/github_pr_comment.py", line 132, in <module>
    skipped_tests = get_skipped_tests_comment(pr_data, github=github)
  File "/home/runner/work/tvm/tvm/ci/scripts/github_skipped_tests_comment.py", line 164, in get_skipped_tests_comment
    common_main_build = get_main_jenkins_build_number(github, common_commit_sha)
  File "/home/runner/work/tvm/tvm/ci/scripts/github_skipped_tests_comment.py", line 57, in get_main_jenkins_build_number
    assert build_number.isdigit()
AssertionError

@driazati @masahi