New flag to prevent loading `bundle exec`

Use `NOEXEC_DISABLE=1` instead of `NOEXEC=0` to skip Bundler.setup, as documented here:
https://github.com/mpapis/rubygems-bundler

If you use NOEXEC=0 as, you get this deprecation warning:
`Warning, 'NOEXEC' environment variable is deprecated,
switch to 'NOEXEC_DISABLE=1'`