Add caching of the yarn dependencies

We're adding a caching mechanism for yarn dependencies built into the
`setup-node` GH action. This should decrease execution time for the step
installing the dependencies.

Author: michalinacienciala

.github/workflows/format.yml

@@ -15,6 +15,7 @@ jobs:
       - uses: actions/setup-node@v2
         with:
           node-version: "14.x"
+          cache: "yarn"
 
       - name: Install dependencies
         run: yarn install