Add --local when setting jh/Gemfile

What does this MR do and why?

When optimizing cross-border bandwidth for JiHu, we found a weird bug that gitaly-rubygems cannot be cached.

It reads from update-setup-test-env-cache job that

Creating cache gitaly-ruby-gems-v1...
WARNING: vendor/gitaly-ruby/: no matching files    
Uploading cache.zip to https://cos.ap-shanghai.myqcloud.com/saas-gitlab-runner-1303695223/gitlab-runner/project/9148/gitaly-ruby-gems-v1 

which is obviously something wrong with bundle path.

We debugged a lot and found the root cause was when we were running bundle config set gemfile jh/Gemfile. It turned out to be a global setting and will affect gitaly. Adding --local will fix the problem.

Now update-setup-test-env-cache job is back to normal

Creating cache gitaly-ruby-gems-v1...
vendor/gitaly-ruby/: found 14819 matching files and directories 
Uploading cache.zip to https://cos.ap-shanghai.myqcloud.com/saas-gitlab-runner-1303695223/gitlab-runner/project/9148/gitaly-ruby-gems-v1 
Created cache

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