actions/setup-ruby does not have 2.6 yet

Author: eregon

.github/workflows/build.yml

@@ -65,9 +65,9 @@ jobs:
 
     # Build
     - uses: actions/checkout@v2
-    - uses: actions/setup-ruby@v1
+    - uses: ruby/setup-ruby@v1
       with:
-        ruby-version: '2.6.x'
+        ruby-version: 2.6
     - name: Download latest jruby nightly archive
       shell: bash
       run: |