Regular development flow is with remote_run.sh, and those packages are easy to install manually if needed.
Also make the environment complete and usable in nix-shell --pure.
Cargo won't let us use the Nix-downloaded copy of zc706 (no way to specify source paths from environment variables,
no way to override Cargo config file without overriding the whole CARGO_HOME) so having such a copy just makes
things complex and inconsistent.
