Pin action version in unix.yml

Signed-off-by: Nigel Jones <[email protected]>

.github/workflows/linux.yml

@@ -259,7 +259,7 @@ jobs:
     container: openquantumsafe/ci-ubuntu-latest:latest
     steps:
       - name: Checkout code
-        uses: actions/checkout@v4
+        uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # pin@v4
       - name: Configure
         run: mkdir build && cd build && scan-build --status-bugs cmake -GNinja ..
       - name: Build