Packaging: pin the MSI test's checkout; the MSI comment says what it installs (#466 follow-up)

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01QiAJeFttZp83Fh7YuzAR5f

.github/workflows/packaging-lint.yml

@@ -73,7 +73,7 @@ jobs:
     runs-on: windows-latest
     timeout-minutes: 15
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
       - name: Install WiX
         shell: pwsh
         run: dotnet tool install --global wix --version 5.0.2