Add SWIFTPM_MODULECACHE_OVERRIDE for Swift CI
Add SWIFTPM_MODULECACHE_OVERRIDE for Swift CI

<rdar://problem/51725297>

abertelrud/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager