Update Homebrew formula to remove support for installing with --head

[wilkinsona]

#33704 is back, this time because of the changes to the build's directory structure.

[wilkinsona]

On further reflection, we think it would be best to drop this feature. It's untested at the moment (hence it being broken without us noticing) and it's also rather brittle. Whenever a change is made to main that affects how the CLI is built, the formula on all maintained branches would then have to be updated to accommodate those changes. That level of overhead doesn't feel worthwhile for a feature that we suspect is barely used. We could reintroduce it in the future if there's sufficient interest.