bug with required provider attributes w/ env variables

I have been trying to make use of this project with [terraform-provider-snowflake](https://github.com/chanzuckerberg/terraform-provider-snowflake) and am very excited for it, since it means I could delete my half-baked code that did similar work.

We have run into one limitation – [required provider configuration which accepts environment variables](https://github.com/chanzuckerberg/terraform-provider-snowflake/blob/main/pkg/provider/provider.go#L26).

When using this configuration, running `terraform providers schema -json` will vary its configuration for these inputs based on whether or not the environment variables are present. For the linked example, if the `SNOWFLAKE_USER` environment variable is set, then `username` will be marked as "optional". If the env variable is not preset, it will be set as "required".

For purposes of documentation, it seems like it should always be marked as "required".