This commit adds a flake.nix build file, it can be used for three
things:
Build `headscale` from local or straight from git:
nix build
or
nix build github:juanfont/headscale
Run and Build `headscale` from local or straight from git:
nix run
or
nix run github:juanfont/headscale
Set up a development environment including all our tools,
- linters
- protobuf tooling
- compilers
nix develop
