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.