maint: Use caching in GitHub actions.

Author: thet

.github/workflows/test.yml

@@ -12,8 +12,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
-    - uses: actions/setup-node@v1
+    - uses: actions/setup-node@v2
       with:
         node-version: '14'
-    - run: npm install yarn
+        cache: 'yarn'
     - run: make check