Add integration test for GHES 3.8+ source

[dylan-smith]

We should probably leave the GHES 3.7 server we use for integration tests in place, as we'll still want to have integration test coverage for the cloud storage scenario.

We should standup a new GHES 3.8 server and add a new integration test that migrates from that to ensure that the new scenario works as intended.

[tambling]

As part of https://github.com/github/octoshift/issues/5741, I've spun up a couple of 3.8 instances, one uploading archives to S3, one to Azure — their creds are in the Octoshift vault in 1Password, as GHES Export Instance (AWS) and GHES Export Instance (Azure), respectively.