CI: pin GitHub Action version

Author: azu

.github/workflows/test.yml

@@ -9,9 +9,9 @@ jobs:
         node-version: [12, 14]
     steps:
       - name: checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@ee0669bd1cc54295c223e0bb666b733df41de1c5 # v2.7.0
       - name: setup Node.js ${{ matrix.node-version }}
-        uses: actions/setup-node@v2
+        uses: actions/setup-node@7c12f8017d5436eb855f1ed4399f037a36fbd9e8 # v2.5.2
         with:
           node-version: ${{ matrix.node-version }}
       - name: Install