chore: remove build files from docker image

- Reduce the size of the docker image
- Make sure ccache is always used => faster CI tests

.github/workflows/conformance.yml

@@ -36,6 +36,7 @@ jobs:
             cd /compiler
             pip install pytest
             rm -rf /build
+            export PYTHONPATH=""
             make CCACHE=ON BUILD_DIR=/build test
             echo "Debug: ccache statistics (after the build):"
             ccache -s