Test classes dependent on environment variables

I have a class which is dependent on an environment variable. I would like to test it with a mock environment variable.

I think a good structure would be to inject the dependency in Spring Boot and then mock it in the test environment. How do I achieve this?

For example with the following code:

@Component
public class ProcessorClass implements Processor {

    public String readFile(String fileName) throws IOException {
            String fileOut;
            String path = Environment.getVariable("READ_PATH");
            fileOut = new String(Files.readAllBytes(Paths.get(path + fileName)));
return fileOut

How would I inject READ_PATH such that I can then mock it within JUnit5.

Environment is a fake class that currently returns a value from System.getenv("...")

Thank you.