ci: Force use of ccache in conformance.yml

Set the make variable `CCACHE` to `ON` in order to force the use of
`ccache` during compilation of the code. For unknown reasons, the
build directory must also be removed. Otherwise, a complete rebuild
circumventing the cache is triggered and `ccache` does not have any
effect on compilation time.

Debug information printing `ccache` statistics before and after the
build has also been added.

.github/workflows/conformance.yml

@@ -31,6 +31,11 @@ jobs:
           password: ${{ secrets.GHCR_PASSWORD }}
           options: -v ${{ github.workspace }}/compiler:/compiler
           run: |
+            echo "Debug: ccache statistics (prior to the build):"
+            ccache -s
             cd /compiler
             pip install pytest
-            make BUILD_DIR=/build test
+            rm -rf /build
+            make CCACHE=ON BUILD_DIR=/build test
+            echo "Debug: ccache statistics (after the build):"
+            ccache -s