This commit adds a [Nix flake] for building Humility for Nix users,
providing a dev environment with build dependencies for use with `nix
develop` or `direnv`, and (on NixOS) configuring udev rules to allow all
debug probes supported by `probe-rs`. Parts of this flake --- in
particular, the code for cross-compiling for multiple targets --- are
based on code written by @faithanalog.

## What The Flake Provides

The flake provides the following:

- Packages for `humility` (for the host platform), as well as
  `humility-aarch64` and `humility-armv7l` for cross-compiling for ARM
  linux environments. These were in @faithanalog's config as I believe
  she's got dev boards hooked up to a Raspberry Pi or something that she
  SSHes into, but would like to build Humility on her dev machine for
  the Pi target.
- The Linux Packages contains the canonical `udev`` rules published by
  `probe-rs`, allowing NixOS users to add
  ```nix
  services.udev.packages = [ pkgs.humility ];
  ```
  to their `configuration.nix`` to allow all the debug probes that
  `probe-rs` supports to be run by non-root users. This config is
  fetched from the `probe-rs/website` Git repository, so if upstream
  adds new supported probes, we can pick up their changes.
- A flake `app` for use with `nix run`, which runs a Humility command.
- A dev shell including Humility's build time dependencies, which can be
  used with `nix develop` or `direnv`
- A NixOS module `programs.humility` which can be used by NixOS users to
  add Humility to the system's packages, add the udev rules, and add
  user account(s) to the `plugdev` group to allow access to debug
  probes.

## On Checking In Nix Configurations

I understand that checking in configurations that are specific to a
package manager/distro that not everyone uses may be controversial. And,
if folks don't want Nix configs in the main Humility repo, that's
understandable, and there are alternative options for Nix users to build
Humility --- we can also have a Nix flake in a separate repo that
fetches the Humility sources from Git. However, checking in this flake
to the Humility repo has some meaningful advantages for Nix users:

1. It allows the Nix build environment for packaging Humility to be used
   as a dev environment for `direnv` or `nix-develop`. If the Nix
   configuration is not in the source tree, it can't easily be used with
   `direnv`. Of course, Nix users can have their own dev environment
   that isn't checked in. However, this would no longer be the *same*
   build environment definition that would be used for building a
   Humility nix package, meaning that there is the potential for drift
   between individual contributors' dev environments and the canonical
   build environment.
2. I'd like to eventually set up publishing to [FlakeHub] on CI when new
   release tags are pushed to the repo, requiring an in-repo Nix flake.
3. I know at least a few folks at Oxide are Nix users (at the very
   least, myself and @faithanalog), and it seems nice to have a
   canonical configuration in the repository rather than requiring folks
   to write their own (potentially differing in minor ways) whenever a
   NixOS user wants to build or contribute to Humility.

If this rationale isn't convincing, that's okay, and I'm happy to close
this PR. :)

[Nix flake]: https://zero-to-nix.com/concepts/flakes
[FlakeHub]: https://flakehub.com/