Allow SSH devices to use the host toolchain

[fredszaq]

This can be quite handy for example in order to test that test resources are properly copied by running it directly on localhost / troubleshooting the remote test exec on a faster machine

Sample configuration in dinghy.toml

[ssh_devices]
hostssh = { hostname = "localhost", username="chucknorris" }

and then

$ cargo dinghy -d hostssh test