BED-4363 fix: missing constraint on roles

[mistahj67]

Related BED-4363 follow-up fix

Description

Identified a missing constraint that breaks the idempotency of v5.15.0

Motivation and Context

This PR addresses: BED-4363

How Has This Been Tested?

Locally ran both and ensured no issues

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Database Migrations

Checklist:

• I have met the contributing prerequisites
  • Assigned myself to this PR
  • Added the appropriate labels
  • Associated an issue: Issues and Pull Requests README #672
  • Read the Contributing guide: https://github.com/SpecterOps/BloodHound/wiki/Contributing
• I have ensured that related documentation is up-to-date
  • Open API docs
  • Code comments (GoDocs / JSDocs)
• I have followed proper test practices
  • Added/updated tests to cover my changes
  • All new and existing tests passed

[urangel]

I tested this out by checking out the v5.14.0 tag and spinning up to get the db in the state at the time. Then I switched to main and started the dev environment and confirmed the fatal error upon running the v5.15.0 migration.

I dropped my volumes rechecked out v5.14.0 tag and spun up to get the db in the state at the time again. This time, I checked out this branch instead of main and confirmed that the migration executed without errors. From what I can tell, this fix checks out. hank you, @mistahj67