Include Homebrew in universal image

I imagine there's a reason this wasn't done already. I thought it's worth opening an issue for others searching too. My use case is replicating my Mac config in Codespaces. I want a few CLIs available everywhere like lazygit. It can be installed with conda-forge but that isn't true for everything. Default Apt sources simply don't include many modern utilities.

Related: microsoft/vscode-dev-containers#357