Rename `build.commandOrigin` to `build.command_origin`

[ehmicky]

@netlify/config returns a build.commandOrigin indicating whether the build command came from the ui or from the config file. This information is used by @netlify/build. Its only effect is to modify how the command is printed on the logs. It is also passed to Bugsnag on build command errors.

Using camelCase is not consistent with the rest of netlify.toml which is using snake_case. Therefore we should rename it to command_origin. We should make sure this is done in a backward compatible way.