.github/workflows/test: temporarily disable jruby-head
.github/workflows/test: temporarily disable jruby-head

`rubocop-ast` chokes on it with:

    TypeError: nil can't be coerced into Integer
        + at org/jruby/RubyFixnum.java:475
        remaining_arities at /home/runner/work/airbrake-ruby/airbrake-ruby/vendor/bundle/jruby/3.1.0/gems/rubocop-ast-1.17.0/lib/rubocop/ast/node_pattern/compiler/sequence_subcompiler.rb:264
        map at org/jruby/RubyArray.java:2794

See: https://github.com/airbrake/airbrake-ruby/runs/5969451901

airbrake/airbrake-ruby,airbrake/airbrake-ruby