fix(ci): correct rubocop condition for Ruby 3.3 in workflow

Fixed the `if` condition that caused the `rubocop` job to not run for Ruby 3.3.
The condition was incorrectly referencing `matrix.ruby` instead of `matrix.ruby-version`, resulting in the step being skipped.

.github/workflows/build.yml

@@ -50,6 +50,6 @@ jobs:
         bundler-cache: true
     - name: Run rubocop
       run: bundle exec rubocop
-      if: ${{ matrix.ruby == '3.3' }}
+      if: ${{ matrix.ruby-version == '3.3' }}
     - name: Run tests
       run: bundle exec rspec