Switch to latest stable LbEnv based on conda and python 3
Fixes:
- More robust setup update with utils/setup.py
Improvements:
- Avoid message "Nothing to be done for `stack.code-workspace'"
- Add a "do not edit" warning in stack.code-workspace
- Update to ccache 3.7.12
Internal changes:
- Explain how CMAKE_PREFIX_PATH is handled
- Use ninja even with Fortran
- Switch to conda-based LbEnv (979)
- Take cmake and ninja from LbEnv (via symlinks)