tools/gcloud: investigate managing via brew cask instead of curl installer

[ooloth]

Context

tools/gcloud/install.bash currently uses the interactive curl installer from cloud.google.com to download the SDK to ~/google-cloud-sdk/. There is an existing TODO in that file noting the Homebrew Cask alternative:

# TODO: replace with brew cask? - https://formulae.brew.sh/cask/gcloud-cli

Questions to investigate

• Does brew install --cask google-cloud-sdk handle gcloud components update the same way, or does brew manage updates entirely (making gcloud components update in update.bash unnecessary)?
• Does the cask install require any post-install steps (auth, cluster credentials, Docker config) that the current install.bash handles manually?
• Would switching to a Brewfile entry (like other tools) let us delete install.bash and simplify update.bash to a brew bundle call?

Goal

If the cask approach is equivalent, replace the curl installer with a Brewfile entry and delete tools/gcloud/install.bash.