Fix build errors on CircleCI

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

```console
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.

```ruby
> 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.

spec/spec_helper.rb

@@ -71,11 +71,11 @@
 
   config.after(:suite) { RuboCop::Cop::Registry.reset! }
 
-  if %w[ruby-head-ascii_spec ruby-head-spec].include? ENV.fetch('CIRCLE_STAGE', nil)
+  if %w[ruby-head-ascii_spec ruby-head-spec].include? ENV.fetch('CIRCLE_JOB', nil)
     config.filter_run_excluding broken_on: :ruby_head
   end
 
-  if %w[jruby-9.3-ascii_spec jruby-9.3-spec].include? ENV.fetch('CIRCLE_STAGE', nil)
+  if %w[jruby-9.3-ascii_spec jruby-9.3-spec].include? ENV.fetch('CIRCLE_JOB', nil)
     config.filter_run_excluding broken_on: :jruby
   end
 end