`bundle_command` changes surprisingly when using a different Gemfile location

Hi :wave:

I had some fun yesterday debugging why my rake commands in the schedule.rb somehow yielded different results than before without having changed anything to whenever.

It turns out, the bundle_command default is determined like so:

https://github.com/javan/whenever/blob/0687268046eaede8f1369a70870e04d3c0c83897/lib/whenever/setup.rb#L26

..which is tied to:

https://github.com/javan/whenever/blob/0687268046eaede8f1369a70870e04d3c0c83897/lib/whenever.rb#L31-L33

Well. My seemingly unrelated change of the day was to move the Gemfile into a different location (say something like Gemfile-ruby33) and point the BUNDLE_GEMFILE env variable to that location. That's not too common, but the standard way to do so (see https://bundler.io/v2.5/man/bundle-config.1.html#LIST-OF-AVAILABLE-KEYS and bundle config gemfile)

I suggest to adjust Whenever#bundler? in a way that works in that scenario as well.

Thanks!