[Commands] Override TMPDIR when shelling out llbuild
[Commands] Override TMPDIR when shelling out llbuild

This creates a new directory under TMPDIR with complete access in the
sandbox.

-- <rdar://problem/33378806> [SR-5491]: LLVM ERROR: IO failure on output stream.
-- https://bugs.swift.org/browse/SR-5061

JGiola/swift-package-manager,abertelrud/swift-package-manager,vmanot/swift-package-manager,apple/swift-package-manager,sschiau/swift-package-manager,sschiau/swift-package-manager,vmanot/swift-package-manager,apple/swift-package-manager,sschiau/swift-package-manager,abertelrud/swift-package-manager,abertelrud/swift-package-manager,vmanot/swift-package-manager,JGiola/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,vmanot/swift-package-manager,apple/swift-package-manager,sschiau/swift-package-manager,JGiola/swift-package-manager,sschiau/swift-package-manager,vmanot/swift-package-manager,abertelrud/swift-package-manager,JGiola/swift-package-manager,abertelrud/swift-package-manager,JGiola/swift-package-manager