refactor go setup

Makefile

@@ -30,9 +30,8 @@ COMMIT?=$(shell git rev-parse HEAD 2>/dev/null)
 ################################################################################
 # ========================= Setup Go With Gimme ================================
 # go version to use for build etc.
-GO_VERSION?=$(shell cat ./.go-version)
 # setup correct go version with gimme
-PATH:=$(shell eval "$$(hack/third_party/gimme/gimme $(GO_VERSION))" && echo "$${PATH}")
+PATH:=$(shell . hack/build/setup-go.sh && echo "$${PATH}")
 export PATH
 # work around broken PATH export
 SHELL:=env PATH=$(PATH) $(SHELL)

hack/build/setup-go.sh

@@ -0,0 +1,29 @@
+#!/bin/bash
+# Copyright 2020 The Kubernetes Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# script to setup go version with gimme as needed
+# MUST BE RUN FROM THE REPO ROOT DIRECTORY
+
+# read go-version file unless GO_VERSION is set
+GO_VERSION="${GO_VERSION:-"$(cat .go-version)"}"
+
+# only setup go if we haven't set FORCE_HOST_GO, or `go version` doesn't match
+# go version output looks like:
+# go version go1.14.5 darwin/amd64
+if ! ([ -n "${FORCE_HOST_GO:-}" ] || \
+      (command -v go >/dev/null && [ "$(go version | cut -d' ' -f3)" = "go${GO_VERSION}" ])); then
+    # eval because the output of this is shell to set PATH etc.
+    eval "$(hack/third_party/gimme/gimme "${GO_VERSION}")"
+fi