Plugin not working after upgrading to SLS v3

**Actual Behaviour**

After updating serverless to v3, I get the following error:

```
npx sls dynamodb migrate --stage test
Environment: darwin, node 14.18.1, framework 3.7.1 (local), plugin 6.1.5, SDK 4.3.2
Docs: docs.serverless.com
Support: forum.serverless.com
Bugs: github.com/serverless/serverless/issues

Error:
Cannot resolve serverless.yml: Variables resolution errored with:
- Cannot resolve variable at "provider.environment": Value not found at "file" source
```

**Expected Behaviour**

Start dynamodb local as usual

**Steps to reproduce it**

Install serverless@3
run `sls dynamodb migrate``

**LogCat for the issue**

**Screenshots of the issue**

**Would you like to work on the issue?**

Reproduce with `npx sls dynamodb migrate --stage test` under Serverless 3.7.1 and inspect how the plugin handles the `serverless.yml` `provider.environment` file variable. Done means the migration command starts DynamoDB Local as expected without the variables-resolution error.

Written by the indexing model from the issue text.