openharmony-standard: Add source mirror to PREMIRRORS
Use PREMIRRORS instead of MIRRORS for source mirror to speed up fetch
jobs.
Signed-off-by: 
Thierry Escande <thierry.escande@huawei.com>
Use PREMIRRORS instead of MIRRORS for source mirror to speed up fetch
jobs.
Signed-off-by: Thierry Escande <thierry.escande@huawei.com>