swift

a1d068de - Call python3 to execute XCTest's build_script.py.

Call python3 to execute XCTest's build_script.py. A workaround for the fact that fixing this in XCTest's repository (https://github.com/apple/swift-corelibs-xctest/pull/441) is blocked on XCTest's CI being broken for unrelated reasons (and ones which I cannot reproduce).