handle GOTOOLCHAIN in kube::golang::verify_go_version

for now:
- shim FORCE_HOST_GO to GOTOOLCHAIN=local
- treat GOTOOLCHAIN set and !=auto like FORCE_HOST_GO
- otherwise set GOTOOLCHAIN=go${GO_VERSION} and fallback to gimme if necessary

TODO: set toolchain statements in go.mod files and keep them in sync

hack/lib/golang.sh

@@ -466,13 +466,23 @@ kube::golang::create_gopath_tree() {
 kube::golang::verify_go_version() {
   # default GO_VERSION to content of .go-version
   GO_VERSION="${GO_VERSION:-"$(cat "${KUBE_ROOT}/.go-version")"}"
-  # only setup go if we haven't set FORCE_HOST_GO, or `go version` doesn't match GO_VERSION
-  if ! ([ -n "${FORCE_HOST_GO:-}" ] || \
-      (command -v go >/dev/null && [ "$(go version | cut -d' ' -f3)" = "go${GO_VERSION}" ])); then
+  if [ "${GOTOOLCHAIN:-auto}" != 'auto' ]; then
+    # no-op, just respect GOTOOLCHAIN
+    :
+  elif [ -n "${FORCE_HOST_GO:-}" ]; then
+    # ensure existing host version is used, like before GOTOOLCHAIN existed
+    export GOTOOLCHAIN='local'
+  else
+    # otherwise, we want to ensure the go version matches GO_VERSION
+    GOTOOLCHAIN="go${GO_VERSION}"
+    export GOTOOLCHAIN
+    # if go is either not installed or too old to respect GOTOOLCHAIN then use gimme
+    if ! (command -v go >/dev/null && [ "$(go version | cut -d' ' -f3)" = "${GOTOOLCHAIN}" ]); then
       export GIMME_ENV_PREFIX=${GIMME_ENV_PREFIX:-"${KUBE_OUTPUT}/.gimme/envs"}
       export GIMME_VERSION_PREFIX=${GIMME_VERSION_PREFIX:-"${KUBE_OUTPUT}/.gimme/versions"}
       # eval because the output of this is shell to set PATH etc.
       eval "$("${KUBE_ROOT}/third_party/gimme/gimme" "${GO_VERSION}")"
+    fi
   fi
 
   if [[ -z "$(command -v go)" ]]; then