When you want to set an environment variable in a Go test, you can use the t.Setenv function.

package main_test

import (
  "testing"

  "github.com/stretchr/testify/assert"
)

func Test_UsingEnvVar(t *testing.T) {
  t.Setenv("ENV_VAR", "value")

  actual := os.Getenv("ENV_VAR")
  assert.Equals(t, "value", actual)
}