docs: append to NODE_OPTIONS, not overwrite

docs/ECMAScriptModules.md

@@ -16,7 +16,7 @@ Also note that the APIs Jest uses to implement ESM support are still [considered
 With the warnings out of the way, this is how you activate ESM support in your tests.
 
 1. Ensure you either disable [code transforms](Configuration.md#transform-objectstring-pathtotransformer--pathtotransformer-object) by passing `transform: {}` or otherwise configure your transformer to emit ESM rather than the default CommonJS (CJS).
-1. Execute `node` with `--experimental-vm-modules`, e.g. `node --experimental-vm-modules node_modules/jest/bin/jest.js` or `NODE_OPTIONS=--experimental-vm-modules npx jest` etc.
+1. Execute `node` with `--experimental-vm-modules`, e.g. `node --experimental-vm-modules node_modules/jest/bin/jest.js` or `NODE_OPTIONS="$NODE_OPTIONS --experimental-vm-modules" npx jest` etc.
 
    On Windows, you can use [`cross-env`](https://github.com/kentcdodds/cross-env) to be able to set environment variables.