Resolve dependencies locally instead of fetching them again remotely

[Edouard-chin]

When writing the Gemfile_next.lock snapshot, we resolve the dependencies remotely https://github.com/Shopify/bootboot/blob/master/lib/bootboot/gemfile_next_auto_sync.rb#L56 this is unnecessary as all the gems should already have been fetched.

Resolving the dependencies locally with Bundler::Definition#resolve_with_cache https://github.com/bundler/bundler/blob/a63a39d738865665f714f179fe42dd10006da26d/lib/bundler/definition.rb#L148 will speed up the install process