First working fhs + hello:

Using only nix + flakes:

nix build github:ES-Nix/fhs-environment/512cce8c11412140eec90fa33186d7673c75714b#fhs-environment
./result/fhs-env -c 'hello'

git clone https://github.com/ES-Nix/fhs-environment.git
cd fhs-environment
git checout 512cce8c11412140eec90fa33186d7673c75714b
nix build .#fhs-environment
./result/fhs-env -c 'hello'

It does not isolate environment variables, why?

nix build .#fhs-environment
./result/fhs-env -c 'env'

This is in two steps:

nix develop --ignore-environment

hello

Don't know how to make it in one step, none of these works:

nix develop --ignore-environment --command hello

nix develop --ignore-environment --command bash -c 'hello'

It may be needed when changing the hardcoded path to the fhs-env: sudo nix-collect-garbage --delete-old