`last_rc` was returning a 6.3.2 release candidate even though 6.3.2 was released

> `last_rc` points to the most recent release candidate. If there is no active release candidate, Bazelisk uses the latest Bazel release instead.

That statement no longer holds, which seems like a bug to me. Hard to test right now, because the 6.4.0 release candidate was just pushed out, but if the bug exists as I understand it, once 6.4.0 proper is released, `last_rc` won't download it.

Locate the implementation of `last_rc` and any tests covering release-candidate selection. Reproduce the behavior after a release candidate becomes a proper release, then verify that `last_rc` uses the latest Bazel release when no active release candidate remains.

Written by the indexing model from the issue text.