Custom Repository role plugin is not working in additive mode

## Problem Description
Added custom repository role plugin in Additive list in settings yaml, it should not delete manually created custom repository roles
### What is actually happening
its deleting all manually created custom repository roles
### What is the expected behavior
it should add a new roles, but should not delete manually created ones
### Error output, if available

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## Context

### Are you using the hosted instance of probot/settings or running your own?

### If running your own instance, are you using it with github.com or GitHub Enterprise?

#### Version of probot/settings

#### Version of GitHub Enterprise