General:
- Rename `unusable_ruby` to `need_vendored_ruby` (due to code changes).
**`setup-ruby-path`** function:
- Move `-z "$HOMEBREW_RUBY_PATH"` and `-n "$HOMEBREW_FORCE_VENDOR_RUBY"` checks to `need_vendored_ruby`.
**`need_vendored_ruby`** function:
- Check `HOMEBREW_FORCE_VENDOR_RUBY` first so that it's respected even
on Macs that are "new enough" (`HOMEBREW_MACOS_SYSTEM_RUBY_NEW_ENOUGH` is set).
- Don't check `-n $HOMEBREW_RUBY_PATH` because it is implicitly tested
by `test_ruby`.
- Remove (now-) redundant `-z $HOMEBREW_FORCE_VENDOR_RUBY` check as it is
tested earlier in the conditional.
15dbb0be58