coreos-cincinnati: allow crates.io in egress policy
    
    This allows reaching crates.io, which is required at build-time
    in order to reach the dependency registry.
    
    Policy change has been acked by puiterwijk:
    https://matrix.to/#/!bcjmqdemgMWRaJJeva:matrix.org/$15611030891834087ceIcE:matrix.org?via=matrix.org&via=disroot.org&via=lisas.de
    
    Signed-off-by: Luca Bruno <luca.bruno@coreos.com>