Test running make html with custom RDOC env var

Author: st0012

.github/workflows/ruby-core.yml

@@ -50,14 +50,13 @@ jobs:
       - uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v3.1.0
         with:
           path: ruby/rdoc
-      - name: Install RDoc locally
+      - name: Build RDoc locally
         run: |
           bundle install
           bundle exec rake build
-          gem install pkg/rdoc-*.gem --install-dir ../ruby/.bundle
         working-directory: ruby/rdoc
       - name: Generate Documentation with RDoc
-        run: make html V=1
+        run: make html V=1 RDOC="$(XRUBY) -I${../rdoc/lib} \"$(tooldir)/rdoc-srcdir\""
         working-directory: ruby/ruby
       # We need to clear the generated documentation to generate them again
       # with the Prism parser.