Install clang-format on ubuntu-slim runner

Unlike the “fat” runners, that's not installed by default.
Why that causes `find` to nonetheless exit normally, I have no idea.

.github/workflows/checkformat.yml

@@ -15,4 +15,5 @@ jobs:
         uses: actions/checkout@v6
       - name: Check format
         run: |
+          sudo apt install clang-format
           contrib/checkformat.bash