ci: update clang to version 6.0 in the Ubuntu build image. (#4157)
ci: update clang to version 6.0 in the Ubuntu build image. (#4157)

*Risk Level*: Low
*Testing*: Successful build using new image.
*Docs Changes*: n/a
*Release Notes*: n/a

Signed-off-by: Piotr Sikora <ba6f3a1a6d7e1ecae91b31e93969cc392fbd0fdd@google.com>
envoyproxy/envoy,dnoe/envoy,envoyproxy/envoy,lizan/envoy,istio/envoy,istio/envoy,lyft/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy-wasm,jrajahalme/envoy,envoyproxy/envoy,dnoe/envoy,lyft/envoy,envoyproxy/envoy,lizan/envoy,envoyproxy/envoy-wasm,lizan/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,eklitzke/envoy,dnoe/envoy,envoyproxy/envoy,istio/envoy,envoyproxy/envoy-wasm,istio/envoy,envoyproxy/envoy,jrajahalme/envoy,jrajahalme/envoy,lizan/envoy,lizan/envoy,dnoe/envoy,lyft/envoy,jrajahalme/envoy,istio/envoy,eklitzke/envoy,envoyproxy/envoy-wasm,eklitzke/envoy,eklitzke/envoy,lyft/envoy,lyft/envoy,istio/envoy,envoyproxy/envoy,dnoe/envoy,envoyproxy/envoy,lyft/envoy,istio/envoy,jrajahalme/envoy,eklitzke/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,envoyproxy/envoy,jrajahalme/envoy,lizan/envoy,envoyproxy/envoy,eklitzke/envoy