10.15 Xcode version: Buildbot, vs. GitHub CI

[Christopher Nielsen]

Just encountered an interesting situation, where a Swift-based port builds successfully via 10.15 CI, but fails on our buildbot.

This appears to be related to Xcode versions: Presently our 10.15 buildbot has Xcode 11.7, whereas 10.15 CI has Xcode 12.4.

This brings up two questions:
* Would it be feasible to update our 10.15 buildbot to a newer Xcode release? Or are there certain ports/situations that necessitate remaining with 11.7?
* Do we have control over the Xcode version used for GitHub CI, or is 12.4 the only option?

Ideally both should utilize the same Xcode 12 release, specifically one that still ships with the 10.15 SDK. So the choices would be 12.0, 12.0.1, and 12.1.

Thoughts?