title: Serverless Framework - Variables resolution error
body: <p>I have serverless.yaml script that use to work before - next after updating to newer version of SLS (2.72.0) I start getting warning:</p>
  <pre><code>Cannot resolve serverless.yaml: Variables resolution errored with:
    - Cannot resolve variable at &quot;custom.S3_BUCKET_NAME&quot;: Value not found at &quot;self&quot; source
  </code></pre>
  <p>my custom section looks like this:</p>
  <pre><code>custom:
    S3_BUCKET_NAME: ${self:service}-data-${opt:stage, self:provider.stage}
    s3Sync:
      - bucketName: ${self:custom.S3_BUCKET_NAME}-website
        localDir: ./dist
        deleteRemoved: true
  </code></pre>
  <p>how I can fix this warning?</p>