Use Yarn caching in GitHub Actions

GitHub actions that 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/add-release-label.yml

@@ -21,6 +21,7 @@ jobs:
         uses: actions/setup-node@v3
         with:
           node-version-file: '.nvmrc'
+          cache: yarn
 
       - name: Install dependencies
         run: yarn --immutable

.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 --immutable