[blunderbuss plugin]: Modify to prevent additional reviewers from being assigned

[bells17]

While it is a rare case, when a PR is created and then changed to draft status, followed by being changed back to undraft status, blunderbuss assigns additional reviewers even though it has already assigned reviewers at the time of PR creation.
Examples of this behavior can be observed in the following PR:

• [ja] Sync the includes folder with en kubernetes/website#46383 (comment)
• [ja] Sync the includes folder with en kubernetes/website#46383 (comment)
• [ja] Sync the includes folder with en kubernetes/website#46383 (comment)

Therefore, it seems necessary to modify the behavior so that additional automatic assignments are not made when the required reviewers have already been assigned.

/assign
/king bug