Pin overlooked action

Author: lloeki

.github/workflows/nix.yml

@@ -41,7 +41,7 @@ jobs:
       - name: Check CPU arch
         run: |
           test "$(uname -m)" = "${{ matrix.platform.cpu }}"
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
         with:
           persist-credentials: false
       - uses: DeterminateSystems/nix-installer-action@e50d5f73bfe71c2dd0aa4218de8f4afa59f8f81d