On NixOS systems, bootstrap will patch rustc used in bootstrapping after
checking `/etc/os-release` (to confirm the current distribution is NixOS).
However, when using Nix on a non-NixOS system, it can be desirable for
bootstrap to patch rustc. In this commit, a `patch-binaries-for-nix`
option is added to `config.toml`, which allows for user opt-in to
bootstrap's Nix patching.
Signed-off-by: NDavid Wood <david.wood@huawei.com>