restore go1.8 image

templates/tools/dockerfile/interoptest/grpc_interop_go1.8/build_interop.sh.template

@@ -1,3 +0,0 @@
-%YAML 1.2
---- |
-  <%include file="../../go_build_interop.sh.include"/>  

tools/dockerfile/interoptest/grpc_interop_go1.8/build_interop.sh

@@ -16,14 +16,14 @@
 # Builds Go interop server and client in a base image.
 set -e
 
-# Turn on support for Go modules.
-export GO111MODULE=on
-
 # Clone just the grpc-go source code without any dependencies.
 # We are cloning from a local git repo that contains the right revision
 # to test instead of using "go get" to download from Github directly.
 git clone --recursive /var/local/jenkins/grpc-go src/google.golang.org/grpc
 
+# Get all gRPC Go dependencies
+(cd src/google.golang.org/grpc && make deps && make testdeps)
+
 # copy service account keys if available
 cp -r /var/local/jenkins/service_account $HOME || true