CI: Add JRuby

Signed-off-by: Takuro Ashie <ashie@clear-code.com>

Author: ashie

.github/workflows/linux.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [ '2.6', '2.7', '3.0' ]
+        ruby: [ '2.6', '2.7', '3.0', 'jruby' ]
         os: [ 'ubuntu-latest' ]
     name: Ruby ${{ matrix.ruby }} unit testing on ${{ matrix.os }}
     steps: