What are the advantages of just nix-build, and adding the result/bin/ to path? Arguably, `nix-env --install` modifies the underlying profiles which manage symlinks. (nix-build + add to PATH is imperative invocation; but `nix-env --install` is and imperative invocation with a stateful effect).

e.g. of "how could that be bad?". I'd managed to get myself into a confusing state with `nix-env --install`. When setting up NixOS, I'd installed firefox as the root user. Months later, in my normal user account, I was confused why such an old firefox was installed (since my profile didn't install firefox). I can't remember exactly, but this also somehow caused problems with fontconfig. -- Probably easily avoidable; but if you don't know what you were doing, you might also try all sorts of install commands and forget.