CI: use coreboot mirror for GNU dependencies of coreboot

CI: use coreboot mirror for GNU dependencies of coreboot

coreboot buildgcc script downloads GNU dependencies from GNU FTP server.
For some reason, this is also as unreliable as their git main server.
There's no option to use a GNU mirror (and I'm not even sure if there's
one for FTP), so we simply pass --mirror to the buildgcc script via the
BUILDGCC_OPTIONS variable so that it makes use of coreboot's mirror.
Hopefully, this proves more reliable than GNU original FTP server.

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>