Configured nix-darwin to write shell configuration to .local
variants (/etc/bash.local, /etc/zshrc.local, /etc/zshenv.local) instead
of managing the main shell files. This allows Salt (corporate laptop
management) to manage /etc/bashrc, /etc/zshrc, and /etc/zshenv while
nix-darwin provides the Nix environment setup through the .local files
that Salt already sources.
