Update GHA depedency (#18)

actions/checkout -> v6.0.2

Author: syncom

.github/workflows/sanity.yml

@@ -11,7 +11,8 @@ jobs:
     runs-on: 'ubuntu-latest'
     steps:
       - name: 'Checkout code'
-        uses: 'actions/checkout@v4'
+        # v6.0.2
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd
       - name: 'Build Docker image'
         run: |
           set -euxo pipefail
@@ -27,7 +28,8 @@ jobs:
     runs-on: 'ubuntu-latest'
     steps:
       - name: 'Checkout code'
-        uses: 'actions/checkout@v4'
+        # v6.0.2
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd
       - name: 'Install shellcheck'
         run: |
           set -euxo pipefail