Use Yarn caching in GitHub Actions

The only GitHub action to install dependencies will now also cache
those dependencies using the standard strategy for Yarn (which is to
hash the lockfile).

This matches the module template (see MetaMask/metamask-module-template#145
for details).

This should have no functional impact except that this action will run
faster when dependencies are unchanged.

.github/workflows/fitness-functions.yml

@@ -18,6 +18,7 @@ jobs:
         uses: actions/setup-node@v3
         with:
           node-version-file: '.nvmrc'
+          cache: yarn
 
       - name: Install dependencies
         run: yarn