sstate: Reduce race windows

sstate: Reduce race windows

When we write to the sstate directory we try to do so atomically so
consumers either see one version or another but never an imcomplete file.
Unfortunately this is reliant on filesystem support and with some NFS
configurations a replaced file would be lost from memory even if users held
open descriptors.

It makes sense to try and avoid replacing existing files where we can.

(From OE-Core rev: 18cdc087fd5da30e2b31f3d4e81b153cd36ca844)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>