build: Fix building in ~/go

Before, build.go only unset GO111MODULE and GOPATH, so the Go compiler
did not see either and worked in Module mode. But if the code is checked
out below ~/go (the default GOPATH), it will detect that the source is
within GOPATH and switch to non-Module mode. Now we're setting
GO111MODULE to "on" explicitly.

build.go

@@ -575,6 +575,9 @@ func main() {
 
 		buildArgs = append(buildArgs, "-mod=vendor")
 		testArgs = append(testArgs, "-mod=vendor")
+
+		goEnv["GO111MODULE"] = "on"
+		buildEnv["GO111MODULE"] = "on"
 	} else {
 		if tempdir == "" {
 			tempdir, err = ioutil.TempDir("", fmt.Sprintf("%v-build-", config.Name))