This assists with build reproducuility. It also avoids errors if cwd
happens not to exist when we call into the fetcher. That situation
would be unusual but I hit it with the unit tests.
(From Poky rev: 86517af9e066c2da1d580fa66b7c7f0340f3403e)
(Bitbake rev: b886c6c15a58643e06ca5ad7a3ff1f7766e4f48c)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>