Check rake build on CI

Sometimes `rake build` is broken. For example in v1.7.2, mismatch
of "cparse-jruby.jar" path between racc.gemspec and Rakefile
makes `rake build` failed.
Add a check on CI to detect such problem in advance.

.github/workflows/test.yml

@@ -41,3 +41,4 @@ jobs:
         ruby-version: ${{matrix.ruby}}
         bundler-cache: true
     - run: bundle exec rake compile test
+    - run: bundle exec rake build