fix: there is an attribute just for bots, let's use it

I noticed this action sometimes doesn't work. And In testing I see the
name for dependabot is not always what we expect. But I see this is_both
attribute so let's use it.

{
  "author": {
    "is_bot": true,
    "login": "app/dependabot"
  }
}

src/action.js

@@ -17,7 +17,7 @@ const run = async () => {
 
     if (
       (!assignees || assignees.length === 0) &&
-      author.login !== 'dependabot[bot]'
+      !author.is_bot
     ) {
       await client.request(
         `POST /repos/${owner}/${repo}/issues/${issue_number}/assignees`,