Fix distcc, data package checkout and more

Fixes:

• Fix distcc for new toolchain wrappers and update docs, closes #54 (closed)
• VSCode: Fix cSpell configuration
• Use custom package checkout and not git-lb-clone-pkg, closes #55 (closed)
• Do not use FETCH_HEAD in staleness check and speed it up

Improvements:

• Explain use of BUILDFLAGS and remove NINJAFLAGS compatibility
• Update to latest stable LbEnv 2129
• Fix localhost links in vscode.md
• Do not remove all files in the Testing directory with make test (preserve cost data)
• "skipped repos" is now a warning at the end (make update)
• Check and warn if utils is behind origin/master
• Build master branch of Detector by default

Internal changes:

• Improve caching of (native) build environment
• Distcc setup improvements
• Add test for $ in ARGS
• Fix CI formatting check
• Add test for distcc support (run manually)