Workaround build task issue in Ruby core CI

The issue is described in ruby/rubygems#8477

The issue fails the workflow whenever RDoc's version is bumped as there
will then be a mismatch between the target file name and the version
that's actually built.

For example:

```
rdoc 6.13.0 built to pkg/rdoc-6.12.0.gem.
```

This commit works around the issue by manually building the gem and
installing it locally so the latest installed version will match the
version contained in the gemspec file.

Author: st0012

.github/workflows/ruby-core.yml

@@ -50,8 +50,13 @@ jobs:
         with:
           path: ruby/rdoc
       - name: Build RDoc locally
+        # The `build` task provided by `require 'bundler/gem_tasks'` seems to have a bug
+        # as described in https://github.com/rubygems/rubygems/issues/8477
+        # The manual `gem build` and `gem install` are used to work around this issue.
         run: |
           bundle install
+          gem build rdoc.gemspec
+          gem install rdoc-*.gem
           bundle exec rake build:local_ruby
         working-directory: ruby/rdoc
       - name: Generate Documentation with RDoc