`snapcraft remote-build` does not support long syntax of the `architectures` top-level key.

This in the recipe:

```
architectures:
  - build-on: amd64
    build-for: amd64
  - build-on: arm64
    build-for: arm64
  - build-on: armhf
    build-for: armhf
  - build-on: ppc64el
    build-for: ppc64el
```

triggers, this:

```
Issues while validating snapcraft.yaml: The 'architectures[0]' property does not match the required schema: additional properties are not allowed ('build-for' was unexpected) or orderedDict([('build-on', 'amd64'), ('build-for', 'amd64')]) is not of type 'string'
```

Which conflicts what the documentation says: <https://forum.snapcraft.io/t/architectures/4972#heading--core22>