build: unexport GOPATH when module vendoring is used

The go compiler doesn't like it when you mix the two, and Travis CI does
exactly that. Take advantage of the opportunity to disable it everywhere
else as well.

Makefile

@@ -8,6 +8,10 @@ export LDFLAGS := -extldflags '$(LDFLAGS)'
 export GCFLAGS := $(FLAGS_ALL)-trimpath '$(PWD)'
 export ASMFLAGS := $(FLAGS_ALL)-trimpath '$(PWD)'
 
+ifneq ($(MOD_VENDOR_ARG),)
+	unexport GOPATH
+endif
+
 MIN_COVERAGE = 89.4
 
 HELP_CMD = \