ci: Skip macOS 'brew update' to save a lot of time

ci: Skip macOS 'brew update' to save a lot of time

Skip macOS 'brew update' before 'brew install' to save up to an hour
of time with Homebrew updates, as per GitHub's recommendations.

See https://github.com/actions/virtual-environments/issues/2173
And https://github.com/microsoft/appcenter/issues/293

This could potentially introduce issues if Homebrew makes a breaking
change in between the 1-2 week window that GitHub Actions updates
their macOS CI images, but it seems unlikely and 'brew update' can be
re-added if needed.