code: func Setenv(ctx context.Context, key, val string) error {
  	if f, ok := ctx.Value(ctxOSSetenvKey).(setenvFunc); ok {
  		return f(key, val)
  	}
  	return os.Setenv(key, val)
  }
docs: // Setenv sets the value of the provided environment variable to the
  // specified value by first inspecting the context for a key "os.Setenv"
  // with a value of func(string, string) error. If the context does not
  // contain such a function then os.Setenv is used instead.