chore(make): specify gnmi target version

Author: abn

Makefile

@@ -2,6 +2,7 @@ PROJECT_ROOT                    ?= gnmi/proto
 PROTO_PACKAGE                   ?= github.com/openconfig/gnmi/proto
 PROTOS                          ?= gnmi_ext/gnmi_ext gnmi/gnmi collector/collector \
                                    target/target
+GNMI_TARGET_VERSION             ?= latest
 
 include Makefile.in
 
@@ -11,7 +12,7 @@ include Makefile.in
 tests/integration/deps:
 	$(call command_check,go)
 	@{ type gnmi_target > /dev/null 2>&1 \
-		|| go install -v github.com/google/gnxi/gnmi_target; \
+		|| go install -v github.com/google/gnxi/gnmi_target@$(GNMI_TARGET_VERSION); \
 	}
 
 ### tests