Activate systemvars to load all system and react .env variables

[sidati]

This tweak made exploitation of variable such as process.env.npm_package_version and any env variable in CRA's .env files possible.

[sidati]

In my case i need to use my package version to perform some conditional actions, and since the cra-append-sw does not have access to process.env.npm_package_version i cannot do that and i have to change the version in both package.json file and my custom servicesWorker.js.

And why doing that while create-react-app is using dotenv-expand react-scripts/config/env.js#L43, so we can take advantage of all the env variables 😁

[chrislim]

I would benefit from this PR too.