Set Gemfile through bundle config

What does this MR do?

1. Add variable GITLAB_BUNDLE_GEMFILE to config Gemfile path required by gitlab-rails bundle;
2. Run bundle config to set Gemfile path when build gitlab rails, config file .bundle/config will be generated under gitlab-rails source directory;
3. Get BUNDLE_GEMFILE through bundle config file which is required in cookbooks;

Related issues

As mentioned in gitlab#338469 (closed), we need a way to specify Gemfile path to make sure we can build correct omnibus package.

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on the dev.gitlab.org mirror for this branch if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened