Use Yarn caching in GitHub Actions (#19662)

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 https://github.com/MetaMask/metamask-module-template/pull/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