webcompat_platform_without_keyword job should also add the keyword to the site report

Per the thread: https://mozilla.slack.com/archives/C4FMB0AJY/p1753293432097889

> We have been adding the webcompat:platform-bug keyword to site reports that are platform bugs. I think it's mostly informative; as long as the site report is blocked on a platform bug we should notice that and treat it as fixable by platform.