Mutation does not work with instrument-cra

Cannot find path to webpack it seems

```
09:21:00 (26962) ERROR DryRunExecutor One or more tests failed in the initial test run:
All tests

> cypress-mutation-example@1.0.0 ci /Users/gleb/git/cypress-mutation-example/.stryker-tmp/sandbox1915589
> start-test 3000 cy:run

1: starting server using command "npm run start"

and when url "[ 'http://localhost:3000' ]" is responding with HTTP status code 200

running tests using command "npm run cy:run"

> cypress-mutation-example@1.0.0 start /Users/gleb/git/cypress-mutation-example/.stryker-tmp/sandbox1915589
> react-scripts -r @cypress/instrument-cra start

/Users/gleb/git/cypress-mutation-example/node_modules/@cypress/instrument-cra/index.js:72
require.cache[webpackConfigPath].exports = fakeConfig
^

TypeError: Cannot set property 'exports' of undefined
at Object. (/Users/gleb/git/cypress-mutation-example/node_modules/@cypress/instrument-cra/index.js:72:42)
```

Start by reproducing the failure with npm run start and npm run cy:run in the cypress-mutation-example project. Inspect node_modules/@cypress/instrument-cra/index.js around line 72 and the reported webpack configuration lookup. Done means the initial test run completes without the TypeError and Cypress mutation testing can proceed.

Written by the indexing model from the issue text.