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)
}