Automatically install libxml2-dev libxslt-dev on TruffleRuby if needed

* This is a special case, because TruffleRuby needs an extra system
  dependency that other Rubies do not, and GitHub runners do not have
  these packages installed by default, while TravisCI has them.

.github/workflows/test.yml

@@ -142,6 +142,19 @@ jobs:
           bundler-cache: true
       - run: bundle exec rails --version
 
+  testTruffleRubyNokogiri:
+    name: "Test installing a Gemfile with nokogiri on TruffleRuby"
+    runs-on: ubuntu-latest
+    env:
+      BUNDLE_GEMFILE: gemfiles/nokogiri.gemfile
+    steps:
+      - uses: actions/checkout@v2
+      - uses: ./
+        with:
+          ruby-version: truffleruby-head
+          bundler-cache: true
+      - run: bundle list | grep nokogiri
+
   lint:
     runs-on: ubuntu-20.04
     steps: