Since the artifacts received by swift-build.py don’t work at the moment, I tried Azure’s GUI again, to find that it works now. I downloaded older artifacts manually and uploaded them to a GitHub release so I could keep experimenting. I guessed wrong and got another incompatible pair, but it did reduce the per‐job download time from 10 minutes to 10 seconds. This whole process would have gone a lot faster if I had realized that sooner.
I also came across this post, which might explain where the need for the C++ standard library comes from. Once I get it compiling again I will try inspecting the linkage of libswiftCore.so.