Ignore the yarn cache directory

Created by: jmorrell

Fixes #2043 (closed)

Test Plan:

❯ create-react-app cra-test
❯ cd cra-test
// simulate a failing test in a yarn cache
❯ mkdir -p .cache/yarn/v1
❯ echo "it('fails', () => throw new Error());" > .cache/yarn/v1/Foo.test.js

Then test that it fails:

Then link the updated version of react-scripts and re-run the test: