248-assumerole added ec2 instance role auth for assume role arn

[bdebyl]

Fixes #248

Proposed Changes

• Modified handling for S3 of Assume Role for case of running Drone pipeline with an EC2 Instance Role

Description

This will allow Drone Cache users running their Drone pipelines within AWS using EC2 Instance Roles to allow authentication for assuming another IAM Role.

Checklist

• [x] Read the CONTRIBUTING document.
• [x] Read the CODE OF CONDUCT document.
• [ ] Add tests to cover changes.
• [x] Ensure your code follows the code style of this project.
• [ ] Ensure CI and all other PR checks are green OR
  • [ ] Code compiles correctly.
  • [ ] Created tests which fail without the change (if possible).
  • [ ] All new and existing tests passed.
• [x] Add your changes to Unreleased section of CHANGELOG.
• [x] Improve and update the README (if necessary).
• [ ] Ensure documentation is up-to-date.