Use cmake for cleaning in cmake job

which makes more sense than building with cmake
and cleaning with make.

.github/workflows/pull-request-checks.yaml

@@ -263,11 +263,9 @@ jobs:
         run: cd build; ctest . -V -L CORE -j${{env.linux-vcpus}}
       - name: Check cleanup
         run: |
+          cmake --build build --target clean
           rm -r build
           rm scripts/bash-autocomplete/cbmc.sh
-          make -C unit clean
-          make -C regression clean
-          make -C jbmc/regression clean
           if [[ $(git status --ignored --porcelain | grep -v .ccache/) ]] ; then
             git status --ignored
             exit 1