Makefile: rubocop --check is not a valid flag (breaks _format)

[matthew-on-git]

Problem

The reference Makefile's `_format` target invokes:

```make
$(call RUBY_EXEC_FOR,rubocop)rubocop --check --fail-level error $$ruby_paths
```

But rubocop has no `--check` flag. Result:

```
invalid option: --check
For usage information, use --help
{"target":"format","status":"fail","duration_ms":1207,"languages":["ruby"],"failed":["ruby"]}
```

`make _format` fails on every Rails project that has rubocop in its Gemfile.

Fix

Default rubocop behavior is already "report, don't autocorrect" — `--check` is unnecessary. `--fail-level error` already gates exit code on errors. Just remove `--check`:

```make
$(call RUBY_EXEC_FOR,rubocop)rubocop --fail-level error $$ruby_paths
```

Filed from a downstream tipsyhive Makefile sync to v1.11.0. Project-side patch applied at sync.