### 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 ```