Fix warning when executing rubocop with bundle exec

`check_commit.rake` requires 'English' but when doing `bundle exec rubocop` this file will also be evaluated.

```
earlopain@DESKTOP-PC rubocop-ast]$ RUBYOPT=-w bundle exec rubocop -V
/home/earlopain/Documents/rubocop-ast/rubocop-ast.gemspec:18: warning: global variable `$RS' not initialized
1.62.1 (using Parser 3.3.0.5, rubocop-ast 1.31.2, running on ruby 3.3.0) [x86_64-linux]
  - rubocop-performance 1.20.2
  - rubocop-rspec 2.27.1
```

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### Bug fixes
+
+* [#288](https://github.com/rubocop/rubocop-ast/pull/288): Fix a Ruby warning (`RUBYOPT=-w`) when doing `bundle exec rubocop` during `rubocop-ast` development. ([@Earlopain][])
+
 ## 1.31.2 (2024-03-08)
 
 ### Bug fixes
@@ -451,3 +455,4 @@
 [@tdeo]: https://github.com/tdeo
 [@ydah]: https://github.com/ydah
 [@sambostock]: https://github.com/sambostock
+[@Earlopain]: https://github.com/Earlopain

rubocop-ast.gemspec

@@ -15,7 +15,7 @@ Gem::Specification.new do |s|
 
   s.email = 'rubocop@googlegroups.com'
   s.files = `git ls-files lib LICENSE.txt README.md`
-            .split($RS) + %w[
+            .split($/) + %w[
               lib/rubocop/ast/node_pattern/parser.racc.rb
               lib/rubocop/ast/node_pattern/lexer.rex.rb
             ]