[README] Use correct build script name
[README] Use correct build script name

The script used to build just XCTest is named "build_script.py", not "build_xctest.py".
davelyon/swift-corelibs-xctest,yoonapps/swift-corelibs-xctest,tiagomartinho/swift-corelibs-xctest,yoonapps/swift-corelibs-xctest,davelyon/swift-corelibs-xctest,apple/swift-corelibs-xctest,apple/swift-corelibs-xctest,tiagomartinho/swift-corelibs-xctest