externalsrc: Fix deltask usage

An incorrect version of the patch merged which entirely removed the covered
variable. This corrects the patch so the code works.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
sstate: Use datastore copies due to data changes persisting

The way the "all arch" PKGSPEC is enabled, it causes corruption
of the datastore of sstate operations against other tasks.

Data store copies are cheap and allow us to use that trick, resetting
to a clean copy of the data afterwards.

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