Use hash for actions/checkout in CI (#13)

Author: adam-fowler

.github/workflows/ci.yml

@@ -56,7 +56,7 @@ jobs:
           POSTGRES_INITDB_ARGS: --auth-host=${{ matrix.postgres-auth }}
     steps:
     - name: Checkout
-      uses: actions/checkout@v6
+      uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd #v6.0.2
     - name: Test
       run: |
         swift test --enable-code-coverage

.github/workflows/nightly.yml

@@ -15,7 +15,7 @@ jobs:
       image: swiftlang/swift:${{ matrix.image }}
     steps:
     - name: Checkout
-      uses: actions/checkout@v6
+      uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd #v6.0.2
     - name: Test
       run: |
         swift test