Install libc++ 8 in istio-builder images. (#1702)
Install libc++ 8 in istio-builder images. (#1702)

Signed-off-by: Piotr Sikora <ba6f3a1a6d7e1ecae91b31e93969cc392fbd0fdd@google.com>
istio/test-infra,istio/test-infra,istio/test-infra,istio/test-infra