oeqa/sdk/cases/buildcpio.py: use gnu mirror instead of main server

oeqa/sdk/cases/buildcpio.py: use gnu mirror instead of main server

ftp.gnu.org is the main server of the GNU project, however download speed
can vary greatly based on one's location.

Using ftpmirror.gnu.org should redirect the request to the closest up-to-date mirror,
which should result sometimes in significantly faster download speed, depending
on one's location. This should also distribute the traffic more across the mirrors.

This information was sourced from https://www.gnu.org/prep/ftp.html

Signed-off-by: Steve Sakoman <steve@sakoman.com>