Fix build errors on CircleCI

[koic]

This PR fixes the following build errors on CircleCI. These tests were expected to be skipped by broken_on: :jruby.

rspec ./spec/rubocop/cli_spec.rb:1632 # RuboCop::CLI configuration from
file fails when a configuration file has invalid YAML syntax
rspec ./spec/rubocop/cop/variable_force_spec.rb:36 #
RuboCop::Cop::VariableForce#process_node when processing a regex with
regopt does not raise an error

https://app.circleci.com/pipelines/github/rubocop/rubocop/6657/workflows/834f8c9c-af93-429c-b434-10b8fa481cdb/jobs/227978

The following is the result of checking by ssh to jruby-9.3-ascii_spec job of CircleCI.

> ENV['CIRCLE_STAGE']
=> nil

> ENV['CIRCLE_JOB']
=> "jruby-9.3-ascii_spec"

CIRCLE_STAGE is not expected to return nil.

As far as I can tell, the description of the official documentation about the CIRCLE_JOB built-in environment variables.

The name of the current job.

https://circleci.com/docs/en/2.0/env-vars

This PR uses CIRCLE_JOB instead of CIRCLE_STAGE to resolve the error.

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.