Makefile/scripts: check and use GOBIN environment variable first

'go get/install' will install binaries into GOBIN when it's set which is not
always same with GOPATH/bin

Fix below error:
$ hack/testExamplesAgainstKustomize.sh HEAD
Installing kustomize HEAD
On linux, and not on remote CI.  Running expensive tests.
make: Nothing to be done for '/home/lizj/gosrc/bin/helm'.
Removing kustomize HEAD
rm: cannot remove '/home/lizj/gosrc/bin/kustomize': No such file or directory

Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>

hack/testUnitKustomizePlugins.sh

@@ -14,6 +14,9 @@ set -o pipefail
 
 rcAccumulator=0
 
+MYGOBIN=$(go env GOBIN)
+MYGOBIN="${MYGOBIN:-$(go env GOPATH)/bin}"
+
 # All hack scripts should run from top level.
 . hack/shellHelpers.sh
 
@@ -49,10 +52,10 @@ function scanDir {
 
 if onLinuxAndNotOnRemoteCI; then
   # Some of these tests have special deps.
-  make $(go env GOPATH)/bin/helmV2
-  make $(go env GOPATH)/bin/helmV3
-  make $(go env GOPATH)/bin/helm
-  make $(go env GOPATH)/bin/kubeval
+  make $MYGOBIN/helmV2
+  make $MYGOBIN/helmV3
+  make $MYGOBIN/helm
+  make $MYGOBIN/kubeval
 fi
 
 for goMod in $(find ./plugin -name 'go.mod' -not -path "./plugin/untested/*"); do