Reintroduce check for go before including bingo Makefile

This re-adds the check for go being installed before including the
bingo variables make file to avoid repeating errors about missing a
missing go binary when running 'make node-generate' in the ci (the node
container doesn't have go installed)

services/gateway/Makefile

@@ -1,7 +1,10 @@
 SHELL := bash
 NAME := gateway
 
+ifneq (, $(shell command -v go 2> /dev/null)) # suppress `command not found warnings` for non go targets in CI
 include ../../.bingo/Variables.mk
+endif
+
 include ../../.make/default.mk
 include ../../.make/recursion.mk
 include ../../.make/go.mk