Some valid `spread.yaml` files don't get parsed correctly.

### Bug Description

Some `spread.yaml` files that spread accepts don't get parsed correctly.

### To Reproduce

Include the example spread snippet below, then run the `test` subcommand.

### part yaml

```shell
backends:
google:
key: '$(HOST: echo "$SPREAD_GOOGLE_KEY")'
location: snapd-spread/us-east1-b
halt-timeout: 3h
systems:
- ubuntu-18.04-64 # String rather than mapping key
```

### Relevant log output

```shell
n/a
```