ci: Bump actions/checkout in cbmc action to v7.0.0

The checkout bump touched only .github/workflows/*, leaving the cbmc
composite action on v6.0.2. Align it with the rest.

Signed-off-by: Matthias J. Kannwischer <matthias@zerorisc.com>

Author: mkannwischer

.github/actions/cbmc/action.yml

@@ -30,7 +30,7 @@ inputs:
 runs:
   using: composite
   steps:
-      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
       - uses: ./.github/actions/setup-shell
         with:
           nix-shell: ${{ inputs.nix-shell }}