brew test-bot --ci-upload is run on a Linux machine.
The macOS bottles have a prefix of "/usr/local".
The default prefix on Linux is /home/linuxbrew/.linuxbrew.
Elide prefix "/usr/local" in the bottle block, even it does not match
the default prefix. Ditto for cellar.
This is the (partially) documented public API for Homebrew.
The main class you should look at is the {Formula} class (and classes linked from there). That's the class that's used to create Homebrew formulae (i.e. package descriptions). Assume anything else you stumble upon is private.