You can't pass dynamic environment to npm run

I know I can add environment variables like this:

task verify(type: NpmTask) {
    environment = ['CYPRESS_BASE_URL' : "http://localhost:9999"]
    args = ['run', 'verify']
}

But in my case the environment variable is only set later on from a custom plugin called "backendServer".
So I tried to add environment in doFirst:

verify.doFirst {
    def randomPort = backendServer.getLocalPort()
    environment = ['CYPRESS_BASE_URL' : "http://localhost:" + randomPort]
}

But this does not work as environment is immutable.

Use the verify NpmTask and its doFirst action as the entry points, and trace how the task's environment is handled when npm run executes. Reproduce the backendServer-generated port scenario and determine how a value assigned later can reach the npm process; done means CYPRESS_BASE_URL uses that runtime port.