add nix flake & direnv support

Adds a nix flake with the following features/enhancements:

• builds docker image without qemu/root
• nix develop environment with compatibility to nix-shell
• direnv support with nix-direnv
• ci infrastructure for docker build and caching
• adds a nix binary cache hosted with cachix
  • pubkey: flow3r.cachix.org-1:/v8059Hm6UdEVNKE15uxltpYM0z+pulaTpobjIvFM5A=
  • currently in the cachix account of @fpletz

The new nix-deps job that runs before all other jobs takes less than one minute if nothing needs to be built in case everything is already cached.

docs/badge/firmware-development.rst

@@ -34,7 +34,7 @@ If you've already cloned without ``--recursive`` you can update your submodules
 Dependencies
 ------------
 
-If you're using Nix(OS), just run ``nix-shell nix/shell.nix``.
+If you're using Nix(OS), just run ``nix-shell nix/shell.nix``. There is also a flake available for ``nix develop``.
 
 On other Linux-based distributions, you will have to manually install ESP-IDF alongside our custom patches (note that install.sh installs stuff to your $HOME so that you may want to use a container or nix):