Reenable JRuby for testing

JRuby's recent issues with jar-dependencies caused installs to
fail, but those issues have been resolved as of JRuby 9.4.12.0
(and by extension, jruby-head).

Reverts part of #1228.

Author: headius

.github/workflows/test.yml

@@ -12,7 +12,6 @@ jobs:
       # 2.7 breaks `test_parse_statements_nodoc_identifier_alias_method`
       min_version: 3.0
       versions: '["mswin"]'
-      engine: cruby-truffleruby
 
   test:
     needs: ruby-versions