Skip protoboeuf-encode on TruffleRuby

Looks like ProtoBoeuf stopped working with TruffleRuby a few months
back when we started using bytesplice:
Shopify/protoboeuf#105

<internal:core> core/string.rb:73:in `bytesplice': wrong number of arguments (given 5, expected 2..3) (ArgumentError)

Author: rwstauner

.github/workflows/test.yml

@@ -16,6 +16,7 @@ jobs:
           - ruby: ruby
           - ruby: head
           - ruby: truffleruby-head
+            skip: protoboeuf-encode
     if: ${{ github.event_name != 'schedule' || github.repository == 'Shopify/yjit-bench' }}
     steps:
       - uses: actions/checkout@v3