test_bot: allow testing portable formulae with other PRs in merge queue

The formulae-detect test currently refuses to run in the merge queue if
any of the PRs in the queue modifies a portable formula. But since the
purpose of this test is to determine bottles to be fetched (to avoid
accidentally merging a PR without publishing bottles), it should be safe
to allow it to run in the merge queue. So let's not abort this test if
we are in the merge queue.

Addresses https://github.com/Homebrew/homebrew-core/pull/284256#issuecomment-4520521431.