Add ssh for nix commands that need ssh e.g. nix-copy-closure

[irisjae]

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Fixes #149.

If detects command needs ssh and a wrapped runtime is selected, installs and provides a suitable ssh binary.

(Perhaps we can come up with a better convention for providing wrapped paths?)

Currently, only detects the nix-copy-closure as needing ssh. Adds NP_SSH environment variable to allow user to add ssh for other commands e.g. nix copy.