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.
