bundle: rename env var to HOMEBREW_BUNDLE_DESCRIBE

Rename HOMEBREW_BUNDLE_DUMP_DESCRIBE to HOMEBREW_BUNDLE_DESCRIBE since
the flag applies to both dump and add subcommands. The old env var still
works with a deprecation warning pointing to the new name.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>