devcontainer: replace `VAULT_HOST` with `AWS_ROLE_ARN`

[jjacobelli]

Description

This PR is replacing the VAULT_HOST variable with AWS_ROLE_ARN. This is required to use the new token service to get AWS credentials.

Checklist

• [ ] I am familiar with the Contributing Guidelines.
• [ ] New or existing tests cover these changes.
• [ ] The documentation is up to date with these changes.

[bdice]

/merge