Anybody know if this is an #opensource install method, or do you need to use https://cinc.sh/ ?

"Install it from source
Note that installing from OS packages from the download page is the preferred method.

That requires bundler:

bundle install
bundle exec bin/inspec help
To install it as a gem locally, run:

gem build inspec.gemspec
gem install inspec-*.gem
On Windows, you need to install Ruby with Ruby Development Kit to build dependencies with its native extensions."