[Fix #10427] Style/For: Mark auto-correction as unsafe

[issyl0]

• The Ruby Style Guide (https://rubystyle.guide/#no-for-loops) states that "for doesn’t introduce a new scope (unlike each) and variables defined in its block will be visible outside it". This means that auto-correcting can break existing, working code because of the change in variable scope.
• Fixes Style/For should be marked as unsafe #10427.

The only thing I didn't do in the PR checklist is add tests, because a recent, different PR to mark a cop as unsafe (10408) didn't have any. Hope that's OK!

---

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.

[issyl0]

Oops, my commit message started [Fixes] instead of [Fix] in the present tense. I force-pushed to amend according to your convention. Looks like someone will have to re-approve the "first time contributor workflow" Actions CI run again, my apologies.

Also, do I have to do anything here to get the CircleCI tests to run?

[jonas054]

I don't understand why the CircleCi runs have not started.

[jonas054]

I think, as a first time contributor, you need to add your name at the bottom of CHANGELOG.md. See the last bullet point on https://github.com/rubocop/rubocop/blob/master/CONTRIBUTING.md.

[koic]

There is no need to manually update the CHANGELOG.md file as first time contributor name will be inserted automatically :-)

[jonas054]

That's great! So much automation going on that I haven't kept up with.

[jonas054]

I've set "approve", since my comment turned out to be unnecessary. @issyl0 Let's hope all the build steps kick off if you push the changes @koic wanted.

[issyl0]

Thanks for the review, @jonas054 and @koic. I made the changes you requested.

[koic]

Thanks!