TBR croupier: Fix the Go test

GOPATH can be empty on Jenkins so instead of appending, we'll just
do a replace.

Change-Id: I22c9ff0381f7d434e51209a87938c6cd18e24aad
diff --git a/go/Makefile b/go/Makefile
index b498a4e..3c59c85 100644
--- a/go/Makefile
+++ b/go/Makefile
@@ -5,7 +5,7 @@
 PATH := ./bin/:$(PATH)
 go_files := $(shell find ./src -name "*.go")
 
-export GOPATH := $(JIRI_ROOT)/release/projects/croupier/go:$(GOPATH)
+export GOPATH := $(JIRI_ROOT)/release/projects/croupier/go
 
 bin/principal:
 	jiri go build -a -o $@ v.io/x/ref/cmd/principal