Add bundle check command to bundle_install script

What does this MR do and why?

Follow up from gem cache implementation with a few further improvements:

• Add bundle check command to skip running bundle install if dependencies are satisfied (rails uses the same in default setup scripts as well)
• Add check for default pg version usage and do not reinstall pg gems in jobs that do not override PG_VERSION variable
• Minor readability improvements for bundle_install_script function
• Set FF_USE_FASTZIP to true for gem cache job: see gitlab-com/gl-infra/delivery#21371 (comment 2658203048)

References

• Ruby caching (gitlab-com/gl-infra/delivery#21371 - closed)

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