force GOTOOLCHAIN="go${GO_VERSION}" for now

TODO: look at using deprecating .go-version in favor of GOTOOLCHAIN existing knobs

hack/build/gotoolchain.sh

@@ -0,0 +1,23 @@
+#!/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 set GOTOOLCHAIN 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)"}"
+
+GOTOOLCHAIN="go${GO_VERSION}"
+export GOTOOLCHAIN GO_VERSION

hack/build/setup-go.sh

@@ -17,7 +17,8 @@
 # MUST BE RUN FROM THE REPO ROOT DIRECTORY
 
 # read go-version file unless GO_VERSION is set
-GO_VERSION="${GO_VERSION:-"$(cat .go-version)"}"
+# override GOTOOLCHAIN unless set as well
+. ./hack/build/gotoolchain.sh
 
 # we don't actually care where the .env files are
 # however, GIMME_SILENT_ENV doesn't trigger re-generating a .env if it