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)