Do not run RuboCop in Container CI

[mishina2228]

This PR stops running RuboCop in Container CI.
It's intended to suppress errors in fedora fedora:rawhide workflow.
https://github.com/brianmario/mysql2/runs/6182290369?check_suite_focus=true

Running RuboCop...
Inspecting 36 files
...............An error occurred while Lint/BlockAlignment cop was inspecting /build/lib/mysql2/console.rb:3:20.
To see the complete backtrace run rubocop -d.
..................An error occurred while Lint/BlockAlignment cop was inspecting /build/tasks/generate.rake:1:0.
To see the complete backtrace run rubocop -d.
...

36 files inspected, no offenses detected

2 errors occurred:
An error occurred while Lint/BlockAlignment cop was inspecting /build/lib/mysql2/console.rb:3:20.
An error occurred while Lint/BlockAlignment cop was inspecting /build/tasks/generate.rake:1:0.
Errors are usually caused by RuboCop bugs.
Please, report your problems to RuboCop's issue tracker.
Mention the following information in the issue report:
0.50.0 (using Parser 2.7.2.0, running on ruby 3.1.2 x86_64-linux)
RuboCop failed!

Since there is already a workflow for RuboCop,
there is no need to run it in Container CI as well.

This is an alternative to #1256.
Once this PR is merged, #1256 can be closed.

[junaruga]

Good point!

[mishina2228]

The errors won't be raised once #1268 is merged,
but excluding the running of RuboCop helps to reduce the time spent on CI.

[sodabrew]

Great suggestion!