feat(cli): add suppot for plugindir as environment variable

pkg/cmd/grafana-cli/main.go

@@ -12,7 +12,9 @@ import (
 var version = "master"
 
 func getGrafanaPluginPath() string {
-	//TODO: try to get path from os:env GF_PLUGIN_FOLDER
+	if os.Getenv("GF_PLUGIN_DIR") != "" {
+		return os.Getenv("GF_PLUGIN_DIR")
+	}
 
 	os := runtime.GOOS
 	if os == "windows" {