I've been trying to use nix-copy-closure to copy the build products, and very strangely an error is complaining that the ssh file doesn't exist. After some debugging, I've determined that most options don't matter, and using a minimal nix-portable nix-copy-closure command triggers the error:
./nix-portable-aarch64 nix-copy-closure --to a@b

error: unable to execute 'ssh': No such file or directory

(Yes, ssh exists on my machine).

I'm running nix-portable-012, and this seems to occur whether I use NP_RUNTIME nix, bwrap, or proot.

This is puzzling, as judging from the error message, the external program call comes from src/libstore/ssh.cc (or more unlikelily, from src/nix/run.cc) of the NixOS/nix repository, and they do use execvpe() which should get ssh from my PATH. I tried adding fake binaries in NP_LOCATION and corresponding PATH entries, and that didn't work either.