Release branches fail CTS checks in CI if CTS is updated in `trunk`

[ErichDonGubler]

RE: CI issues noticed by @aleiserson after merging #8311 (comment link):

(The problem is that I bumped the CTS version on trunk earlier today, and the CTS is cloned with --depth 1. So the cache from trunk will not work for the v27 branch. Certainly there's a better fix but changing the tag should suffice for now.)