Updating the toolchain doesn't update profile fields

[david-yan]

Running oasis set-toolchain latest doesn't update the fields in the profile. This leads to unexplained and undocumented errors such as
error: `profile.local` is missing field: `gateway`.
when trying to deploy. We should either make sure that the update updates the fields, or there's documentation somewhere explaining what to do about this error.

[nhynes]

This was a one time breaking change. If we ever do this again (I hope we don't), we'll update the fields.