Storing the du output in a variable seemed convenient, but I didn't
realise just how big it could get especially with things like infra
images -- there's something like 100MiB of text being stored in a bash
variable here.
Convert this to work with a temporary file
Change-Id: I6a6d22c2142e0f199490c39cca8c94769e4b0232
48daefb685