Run "bundle install" before cd-ing into browser folder

Once deployed, need to have related commits merged:

https://gerrit.wikimedia.org/r/#/q/topic:bug/69245,n,z

Preparing for running RuboCop after every push to Gerrit. Bundler and
RuboCop configuration files are now located in the repository root.

Minor but breaking change to all browsertests-* jobs.

Bug: 69245
Change-Id: Id2676255656c8f03ac9bb31ea1959775f96ef55e