Create roles/common.nix containing shared configuration between NixOS and
Darwin: timezone, base packages (git, glances, pciutils, tree, usbutils, vim),
nix settings (experimental-features, max-jobs, trusted-users), gc config,
and allowUnfree setting.
Both roles/default.nix and roles/darwin.nix now import common.nix and only
contain platform-specific configuration.
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.
