Chore: Fix Makefile call to linter.getRules()

[btmills]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[x] Other, please explain: Fix the release script

What changes did you make? (Give an overview)

The first attempt to release v8.0.0-beta.0 failed when Makefile.js tried to call linter.getRules().

eslint/Makefile.js

Line 641 in 0c86b68

const rules = require(".").linter.getRules();

The linter object was removed in v8.0.0, so we need to create an instance of Linter.

Is there anything you'd like reviewers to focus on?

There's another getRules() call later in the script when it's generating formatter examples, but that's called from CLIEngine, so it should work. At some point we ought to migrate Makefile.js to the ESLint API.