Allow assumption of the Users account provision role

[jsf9k]

🗣 Description

This pull request modified the Terraform code to allow COOL assessment provisioners to also assume the Users account provision role.

See also cisagov/cool-assessment-terraform#254.

💭 Motivation and context

This is necessary due to the changes in cisagov/cool-assessment-terraform#254.

🧪 Testing

All automated tests pass. I also deployed these changes to our COOL Users account and verified as part of the testing for cisagov/cool-assessment-terraform#254 that they function as expected.

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All new and existing tests pass.