Run test-bundler / test-bundled-gems for Ubuntu

only once. Doing it on multiple Ubuntu versions may not worth it.

.github/workflows/ubuntu.yml

@@ -12,6 +12,11 @@ jobs:
       matrix:
         os: [ubuntu-latest, ubuntu-16.04]
         test_task: [ "check", "test-bundler", "test-bundled-gems" ]
+        exclude:
+          - os: ubuntu-16.04
+            test_task: test-bundler
+          - os: ubuntu-16.04
+            test_task: test-bundled-gems
       fail-fast: false
     runs-on: ${{ matrix.os }}
     if: "!contains(github.event.head_commit.message, '[ci skip]')"