Assume an AWS role in Evergreen so that Compass CI can S3 GET the private -latest server artifacts.

• Use ec2.assume_role: https://docs.devprod.prod.corp.mongodb.com/evergreen/Project-Configuration/Project-Commands#ec2assume_role
• Every task assumes the role, not only the latest-alpha one because more downloads might to move to S3 later.

In Compass this touches .evergreen/functions.yml and the test-latest-alpha variant defined in .evergreen/buildvariants-and-tasks.in.yml (see TEST_LATEST_ALPHA_SERVER_VERSION around L160 and the task around L268).