Pin actions/checkout to SHA and disable credential persistence

Pin actions/checkout to v7.0.0 SHA (9c091bb) to mitigate supply-chain
risk from mutable tags. Add persist-credentials: false since the
workflow only needs read access for shellcheck validation.

Author: sebrandon1

.github/workflows/shellcheck.yml

@@ -18,7 +18,9 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+        with:
+          persist-credentials: false
 
       - name: Run ShellCheck
         run: make test-shellcheck