flavours: disable public sstate cache by default

The cache is relatively network-heavy, as it attempts to check if one of
myriad of cache files is available and is somewhat misleading, since the
cache only works for the small subset of configurations that have been
cleared for legal re-distribution and have a public cache available.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>