Can't access 'env' properties in nested bindings

[YoniSegal]

If I have a jenkins shared library which looks like this for example:

└── project
    ├── src
    │   ├── helper.groovy
    │   └── mainFunc.groovy
    ├── test
    │   └── mainFuncTest.groovy
    └── vars
        └── myStep.groovy

src/helper.groovy

def call() {
    println(env.BUILD_URL)
}

src/mainFunc.groovy:

def call() {
    println(env.BUILD_URL)
    def helper = new helper()
    helper()
}

vars/myStep.groovy

def call() {
    def mainFunc = new mainFunc()
    return mainFunc
}

test/mainFuncTest.groovy:

import com.lesfurets.jenkins.unit.BasePipelineTest
import org.junit.Before
import org.junit.Test

class mainFuncTest extends BasePipelineTest {

    @Override
    @Before
    void setUp() throws Exception {
        baseScriptRoot = 'project'
        scriptRoots += 'src'
        scriptExtension = 'groovy'
        super.setUp()
    }

    @Test
    void test1() {
        Script mainFunc = loadScript("mainFunc.groovy")
        addEnvVar("BUILD_URL", "http://something")
        mainFunc()
    }
}

When running test1, I get the expected print: http://something, however, when mainFunc() calls helper(), I get an error message: No such property: env for class: helper

I understand that when running addEnvVar("BUILD_URL", "http://something") in test1, it adds env.BUILD_URL as a property to mainFunc's binding.

Is there anyway to make the env property truly global? Such that it would be accessible in helper's binding?

If it's not possible, can someone please explain how to mock helper.call?

Thank you