Add workaround for CI failure (#742)

The system on CI rake version is 13.0.6, while the bundler's rake version is 13.1.0.
IRB's test depends on the system's rdoc version of rake.
Match the rake version installed by bundler to the version installed on the system.

.github/workflows/test.yml

@@ -72,6 +72,7 @@ jobs:
       - name: Install dependencies
         run: |
           gem install bundler --no-document
+          gem install rake
           gem rdoc --all --ri --no-rdoc
           WITH_VTERM=1 bundle install
       - name: rake test_yamatanooroti