Set the homebrew variables to avoid uncessary network access

[samoht]

For the CI, it's necessary to avoid network access when possible (especially when we know when things are already installed). homebrew allows users to set a few environment variables to control this; Right now, opam-repository does this (although not always properly but it would be nice if opam-depext would do this instead.

The variables are to set are:

  HOMEBREW_NO_INSTALL_FROM_API = 1
  HOMEBREW_NO_AUTO_UPDATE = 1

[kit-ty-kate]

A notable side note to that is that: this behaviour is a bug that only exists for Homebrew 4.0.0 and 4.0.1. So most users shouldn't hit this.