Set GOCACHE to go-build directory inside builddir

for the extra build command outside of dh-golang's control
to avoid build error with buildd running sbuild.

debian/rules

@@ -7,7 +7,9 @@
 
 override_dh_auto_build:
 	dh_auto_build
-	GOPATH=$(shell pwd)/obj-$(DEB_HOST_GNU_TYPE) make secretservice
+	GOPATH=$(shell pwd)/obj-$(DEB_HOST_GNU_TYPE) \
+	GOCACHE=$(shell pwd)/obj-$(DEB_HOST_GNU_TYPE)/go-build \
+	       make secretservice
 
 override_dh_auto_clean:
 	dh_auto_clean