build fails when schemaType is set in porter.yaml

[carolynvs]

We documented that schemaType was supported for the porter manifest, see https://getporter.org/bundle/manifest/file-format/#example, but when you actually set it, porter fails to validate the manifest because it's an unexpected field.

$ porter build
Using buildkit build driver
unsupported property set or a custom action is defined incorrectly: error unmarshaling custom action schemaType: yaml: unmarshal errors:
  line 1: cannot unmarshal !!str `Bundle` into manifest.Steps

This is on porter v1.0.0-rc.1

Reproduction Steps:

1. Run porter create
2. Edit the bundle and include schemaType: Bundle at the top
3. Run porter build

[carolynvs]

For the v1.0.0 release, I'll fix the docs so that people know that schemaType isn't supported on porter.yaml. Then in a patch release post v1, we'll fix this along with #2269