Declare Homebrew through nix-darwin instead of Brewfile

Declare Homebrew through nix-darwin instead of Brewfile Ports the Brewfile into nix/modules/homebrew.nix: 22 casks, 7 Mac App Store apps, and 2 formulae. Verified by diffing the evaluated cask list against `brew list --cask` - the only difference is the font, below. Cleanup is deliberately "none" rather than "zap". Zap is the equivalent of the old `make brew-clean-force`, but it runs on every activation instead of on demand and removes application support and preference files along with the app. It gets tightened once these lists have survived real switches. autoUpdate and upgrade are both off so activation stays fast and deterministic; upgrading is a deliberate act and stays in `make update`. Two changes to what Homebrew carries: - font-fira-code-nerd-font becomes fonts.packages = [ nerd-fonts.fira-code ], so the cask is dropped here. The cask is still installed on this machine and cleanup is off, so nothing is removed until it is uninstalled by hand. - sshpass leaves Homebrew in a later commit, which is why the hudochenkov/sshpass tap is not declared. That also retires `brew-trust`. Deleting the Brewfile breaks `make update`, which still points at it. `make install` has been broken since the rename commit; both are rewritten when the Makefile is cut down.