Add cli main & user conf option for 'dont-cache-buildtrees' context

_context.py: Add dont_cache_buildtrees global user context, the
default of which is set to by default to False via the addition of
dont-cache-buildtrees to userconfig.yaml cache group.

_frontend/app.py & cli.py: Add --dont-cache-buildtrees as a bst
main option, which when passed will override the default or user
defined context for dont_cache_buildtrees.

tests/completions/completions.py: Update for the added flag.