Now that which-formula lives in this repo, we should fetch executables.txt on brew update invocations. When a user runs brew which-formula for the first time, a setting is saved indicating that executables.txt should be downloaded on future brew update calls. If a user does not run brew which-formula, the file will not be fetched.

To make this easier, I've extracted the API file fetch logic to a helper function in update.sh. You can view that change in isolation in 1f042a3 (#20739).

I've also added --skip-update to the shell handlers that call which-formula so no new files will be downloaded if a user types the wrong command in their shell.