Add a simple shell.nix to enable QMK toolchain in shell

[evantravers]

Solves: https://github.com/qmk/qmk_userspace/issues/8

Description

[budimanjojo]

Currently I have a flake.nix working locally and I don't use the qmk provided by the stable/unstable nixpkgs repo because they get outdated most of the time. So I use buildPythonApplication to build the package manually because it allows more control on the qmk version. The downside is needing to build the program in the environment and updating this file frequently is needed. But if you want to do it this way, I propose using the unstable branch instead of pinning it to 24.05. Because this will get outdated very fast.