project: add Nix Flake
This allows easily building and running the testing CLI on any machine having Nix installed, automatically using the pinned version of the Rust toolchain.
This is especially useful for running computation-intensive tests on a remote
machine by simply using something like:
nix run hg+https://foss.heptapod.net/mercurial/vcsgraph#testing -- --help
For now, only the x86_64 variant of the derivation is defined, because I do not have other systems for testing (yet).