We use FNM instead of NVM. Reason: the setup seems easier with just
`eval "$(fnm env)"`.
So now, we try to install Node.js; if the command fails, it could be
because FNM isn't installed (and we need to install it) or because the
version of Node.js cannot be installed with the current version of FNM
(and we need to update FNM). After installing/updating FNM, we try to
install Node.js again.
Note we're using `fnm env` in the middle of the `fnm_setup_command`.
That way, the command will raise a `SSHKit::Command::Failed` exception
if `fnm` isn't installed, and it will give us an indicator that we need
to actually install it.
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