ci: select gcov-7 for consistency with gcc-7 in Docker image. (#6176)
ci: select gcov-7 for consistency with gcc-7 in Docker image. (#6176)

This is needed to enable Bazel native coverage (without, we don't get
coverage files generated).

Risk level: Low
Testing: bazel coverage run inside CI image.

Signed-off-by: Harvey Tuch <c185081708e02336640916420d0c772668570de5@google.com>
jrajahalme/envoy,eklitzke/envoy,dnoe/envoy,istio/envoy,envoyproxy/envoy,eklitzke/envoy,envoyproxy/envoy,eklitzke/envoy,envoyproxy/envoy-wasm,istio/envoy,lyft/envoy,envoyproxy/envoy-wasm,istio/envoy,eklitzke/envoy,dnoe/envoy,envoyproxy/envoy,lizan/envoy,istio/envoy,envoyproxy/envoy-wasm,lyft/envoy,lyft/envoy,lyft/envoy,dnoe/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy-wasm,eklitzke/envoy,envoyproxy/envoy,lizan/envoy,envoyproxy/envoy,lyft/envoy,jrajahalme/envoy,envoyproxy/envoy,jrajahalme/envoy,lizan/envoy,lizan/envoy,jrajahalme/envoy,lizan/envoy,envoyproxy/envoy,dnoe/envoy,istio/envoy,lizan/envoy,dnoe/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,jrajahalme/envoy,envoyproxy/envoy,lyft/envoy,istio/envoy,jrajahalme/envoy,envoyproxy/envoy,envoyproxy/envoy,eklitzke/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,istio/envoy