nh picks remote elevation command based on what's available locally

[faukah]

This now "works for me" (sudo + run0-sudo-shim) but it isn't quite fixed because nh still picks the elevation command based on what's available locally.

For example:

1. The local machine uses sudo.
2. The remote machine uses doas.

nh will:

1. Pick sudo in EllevationStrategy::resolve because that's what's available locally.
2. Refuse to use anything else, even when attempting to force a specific method, because which will never be able to find the requested command.

Originally posted by @Stebalien in #556