I just made a new build for the 4.9 branch and then I tried upgrading to it via WP-CLI, which I had used to upgrade to a nightly branch build previously. WP-CLI tried to use the previously-cached ZIP:

$ wp core upgrade --force http://wordpress.org/nightly-builds/wordpress-4.9-latest.zip
Starting update...
Using cached file '/root/.wp-cli/cache/core/wordpress-4.9-latest-.zip'...
Unpacking the update...

Obviously this is not what I wanted. So I had to add a cache-busting query parameter to force it to skip the cache.

I think that if the URL contains “nightly-builds” then the caching should be bypassed.

Related: #17