CLI-supplied environment variables can't contain a comma

[na--]

Running the following script:

export default function () {
    console.log(JSON.stringify(__ENV));
}

with k6 run --include-system-env-vars=false --env key="val1,val2" script.js will result in {"key":"val1","val2":""} being printed...

It seems like the pflag.StringSliceP() function is trying to be helpful and split the values when there's a comma and we should use the StringArrayP(), which doesn't do that.