Added .eslintcache caching to CI

refs https://github.com/TryGhost/Toolbox/issues/532

- this will cache the `.eslintcache` files in packages, which should
  dramatically speed up linting in CI

.github/workflows/test.yml

@@ -26,6 +26,12 @@ jobs:
         with:
           node-version: '16.13.0'
           cache: yarn
+
+      - uses: actions/cache@v3
+        with:
+          path: ghost/**/.eslintcache
+          key: eslint-cache
+
       - run: yarn
       - run: yarn lint
       - uses: daniellockyer/action-slack-build@master