ci: fix Debian-10 (Buster) install instructions (#105)

Only noticeable in g-c-cpp and g-c-cpp-spanner, but we need the
development version of libc-ares as we use this library indirectly (via
gRPC).

Author: coryan

INSTALL.md

@@ -598,7 +598,7 @@ Google Cloud Platform proto files. We simply install these pre-built versions:
 
 ```bash
 sudo apt update && \
-sudo apt install -y libgrpc++-dev libprotobuf-dev \
+sudo apt install -y libgrpc++-dev libprotobuf-dev libc-ares-dev \
         protobuf-compiler protobuf-compiler-grpc
 ```
 

ci/kokoro/install/Dockerfile.debian-buster

@@ -37,7 +37,7 @@ RUN apt update && \
 
 # ```bash
 RUN apt update && \
-    apt install -y libgrpc++-dev libprotobuf-dev \
+    apt install -y libgrpc++-dev libprotobuf-dev libc-ares-dev \
         protobuf-compiler protobuf-compiler-grpc
 # ```
 

ci/templates/kokoro/install/Dockerfile.debian-buster.in

@@ -33,7 +33,7 @@ RUN apt update && \
 
 # ```bash
 RUN apt update && \
-    apt install -y libgrpc++-dev libprotobuf-dev \
+    apt install -y libgrpc++-dev libprotobuf-dev libc-ares-dev \
         protobuf-compiler protobuf-compiler-grpc
 # ```