`path add`, when given a record, sets `$env.PATH` according to the value
of the key matching `$nu.os-info.name`. There already existed a check in
place to ensure the correct column existed, but it was never reached
because of an early error on `path expand`ing `null`. This has been
fixed, as well as the out-of-date reference to "darwin" instead of
"macos" in the example.
# User-Facing Changes
`path add` now simply ignores a record that doesn't include a key for the current OS
`path add` also will no longer add duplicate paths.