[6.0] Set environment variable to disable building swift-syntax in Swift 6 mode

[ahoppen]

Companion of swiftlang/swift-syntax#2627

• Explanation: This allows us to work around the following issue in CI: The self-hosted SwiftPM job has Xcode 15.3 (Swift 5.10) installed and builds a Swift 6 SwiftPM from source. It then tries to build itself as a fat binary using the just-built Swift 6 SwiftPM, which uses xcbuild from Xcode as the build system. But the xcbuild in the installed Xcode is too old and doesn't know about Swift 6 mode, so it fails with: SWIFT_VERSION '6' is unsupported, supported versions are: 4.0, 4.2, 5.0 (rdar://126952308). This is fixed by setting SWIFTSYNTAX_DISABLE_SWIFT_6_MODE in build-using-self.
  The source compat suite is seeing the same issue, just by using bootstrap, so set it there as well.
• Scope: The build of SwiftPM using the bootstrap and build-using-self scripts
• Risk: All issues that might have been caused by this will be found at build time
• Testing: Verified that both the self hosted Swift CI job and the source compat suite build fine with Set environment variable to disable building swift-syntax in Swift 6 mode #7495
• Issue: rdar://125579439
• Reviewer: @MaxDesiatov on Set environment variable to disable building swift-syntax in Swift 6 mode #7495

[ahoppen]

swiftlang/swift-syntax#2627

@swift-ci Please test