kubectl plugin: Expand environment variables in the command of plugin.yaml

This commit improves kubectl plugins to expand environment variables in
the command of plugin.yaml.

pkg/kubectl/plugins/runner_test.go

@@ -18,6 +18,7 @@ package plugins
 
 import (
 	"bytes"
+	"os"
 	"testing"
 )
 
@@ -38,8 +39,16 @@ func TestExecRunner(t *testing.T) {
 			command:     "false",
 			expectedErr: "exit status 1",
 		},
+		{
+			name:        "env",
+			command:     "echo $KUBECTL_PLUGINS_TEST",
+			expectedMsg: "ok\n",
+		},
 	}
 
+	os.Setenv("KUBECTL_PLUGINS_TEST", "ok")
+	defer os.Unsetenv("KUBECTL_PLUGINS_TEST")
+
 	for _, test := range tests {
 		outBuf := bytes.NewBuffer([]byte{})