Fix caching of bundled gems in the Ruby.gitlab-ci.yml

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What does this MR do and why?

In the default Ruby.gitlab-ci.yml template exists an incorrect configuration for caching of bundled gems, as a result, the consequent commits and triggered CI pipelines fetch and build Ruby gems instead of reading them from the cache:

$ bundle config set --local deployment true
$ bundle install -j $(nproc)
Fetching rake 13.2.1
Installing rake 13.2.1
Fetching bigdecimal 3.1.7
Fetching base64 0.2.0
Installing base64 0.2.0
Fetching concurrent-ruby 1.2.3
...

After this small fix, it works faster because reads gems from the cache:

$ bundle config set --local deployment true
$ bundle config set --local path './vendor/ruby'
$ bundle install -j $(nproc)
Using rake 13.2.1
Using base64 0.2.0
...

MR acceptance checklist

It speeds up automated tests passing

Screenshots or screen recordings

Before: https://gitlab.com/abratashov/kamal-blog-github/-/jobs/6646173877

After: https://gitlab.com/abratashov/kamal-blog-github/-/pipelines/1256617021

How to set up and validate locally

1. Just create two PRs with previous and current CI changes and check timing of pipeline running, e.g. https://gitlab.com/abratashov/kamal-blog-github