Update actions/checkout and pin

Author: nathanjmcdougall

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
       - name: Checkout code
-        uses: actions/checkout@v2
+        uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
 
       - name: Setup uv and handle its cache
         uses: hynek/setup-cached-uv@49a39f911c85c6ec0c9aadd5a426ae2761afaba2 # v2.0.0