dev: Add support for NixOS dev environment

[MonsterDruide1]

Allows building the project on NixOS environments and fixes all (?) places where dependencies and external inputs are used.

Notes:

• to enable nix / load the flake, a .envrc.private containing export USE_NIX=true must be created, otherwise it's disabled by default
• a new workflow has been added accordingly, which checks whether the build on NixOS still succeeds as expected. It takes about 30 minutes to execute, as it builds all dependencies without cache, making sure that every workflow starts fresh and errors are caught the moment they are introduced.
• stack.yaml must also pin the nixpkgs version, as packages and the compiler are installed there instead of through the flake
• makefile has been fixed to also add ATTACH_RESOURCE_ZIP=1 on the standard build target