Build TruffleRuby 20.3.0

Author: eregon

.github/workflows/build.yml

@@ -6,12 +6,12 @@ on:
 jobs:
   # Build stable releases
   build:
-    if: false
+    if: true
     strategy:
       fail-fast: false
       matrix:
         os: [ ubuntu-16.04, ubuntu-18.04, ubuntu-20.04, macos-latest ]
-        ruby: ["ruby-3.0.0-preview1"]
+        ruby: ["truffleruby-20.3.0"]
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v2