refactor: set default environment variables format in plugin (#253)

- Add `INPUT_ENVS_FORMAT` to the list of environment variables in `main.go`
- Define `envsFormat` variable in `plugin.go`
- Add default value for `Config.EnvsFormat` in `plugin.go` `Exec()` function

fix https://github.com/appleboy/drone-ssh/pull/235

main.go

@@ -199,8 +199,8 @@ func main() {
 		&cli.StringFlag{
 			Name:    "envs.format",
 			Usage:   "",
-			EnvVars: []string{"PLUGIN_ENVS_FORMAT"},
-			Value:   "export {NAME}={VALUE}",
+			EnvVars: []string{"PLUGIN_ENVS_FORMAT", "INPUT_ENVS_FORMAT"},
+			Value:   envsFormat,
 		},
 	}