Add PR check job for Ruby beta

Add a separate job that analyses the multilanguage test repo's Ruby code.
For now, run this only with the latest released CodeQL build from defaults.json.
The cached builds and nightly builds don't support Ruby yet.

In future, we can update this and other PR checks to test a wider range of cases.

.github/workflows/pr-checks.yml

@@ -497,6 +497,47 @@ jobs:
           exit 1
         fi
 
+  # Ruby is in beta, so test it separately for now.
+  multi-language-repo_test-ruby:
+    needs: [check-js, check-node-modules, check-codeql-versions]
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
+        tools:
+          - latest
+          # TODO: Uncomment when nightly builds also support Ruby in beta.
+          # - ${{ needs.check-codeql-versions.outputs.nightly-url }}
+    runs-on: ${{ matrix.os }}
+    env:
+      CODEQL_ENABLE_EXPERIMENTAL_FEATURES: true
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Move codeql-action
+      shell: bash
+      run: |
+        mkdir ../action
+        mv * .github ../action/
+        mv ../action/tests/multi-language-repo/{*,.github} .
+        mv ../action/.github/workflows .github
+    - uses: ./../action/init
+      with:
+        languages: ruby
+        tools: ${{ matrix.tools }}
+    - uses: ./../action/analyze
+      id: analysis
+      env:
+        TEST_MODE: true
+    - name: Check database
+      shell: bash
+      run: |
+        RUBY_DB="${{ fromJson(steps.analysis.outputs.db-locations).ruby }}"
+        if [[ ! -d "$RUBY_DB" ]]; then
+          echo "Did not create a database for Ruby."
+          exit 1
+        fi
+
   multi-language-repo_rubocop:
     needs: [check-js, check-node-modules]
     runs-on: ubuntu-latest