Split JRuby CI runs from MRI CI runs

We can improve the performance of our MRI test runs by not installing
Java as part of their run. (Java is only required for JRuby.)

.github/workflows/test.yaml

@@ -1,11 +1,12 @@
 name: Ruby Build
 on: [push, pull_request]
 jobs:
-  build:
+  jruby_build:
+    name: JRuby (${{ matrix.ruby }})
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: [2.4.0, 2.6.0, 3.0.0, jruby-9.2.18.0]
+        ruby: [jruby-9.2.18.0]
     steps:
       - run: echo "The job was automatically triggered by a ${{ github.event_name }} event."
       - run: echo "This job is now running on a ${{ runner.os }} server hosted by GitHub!"
@@ -21,9 +22,33 @@ jobs:
         with:
           distribution: 'adopt'
           java-version: '11'
+      - name: Set up Ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: ${{ matrix.ruby }}
+          bundler-cache: true
+      - name: Run tests
+        run: bundle exec rake
+  mri_build:
+    name: Ruby (${{ matrix.ruby }})
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        ruby: [2.4.0, 2.6.0, 3.0.0]
+    steps:
+      - run: echo "The job was automatically triggered by a ${{ github.event_name }} event."
+      - run: echo "This job is now running on a ${{ runner.os }} server hosted by GitHub!"
+      - run: echo "The name of your branch is ${{ github.ref }} and your repository is ${{ github.repository }}."
+      - name: Check out repository code
+        uses: actions/checkout@v2
+      - run: echo "The ${{ github.repository }} repository has been cloned to the runner."
+      - run: echo "The workflow is now ready to test your code on the runner."
+      - name: List files in the repository
+        run: |
+          ls ${{ github.workspace }}
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby }}
           bundler-cache: true
-      - name: exec rake
+      - name: Run tests
         run: bundle exec rake