Avoid failing CI on TruffleRuby

[k0kubun]

The current truffleruby-head doesn't seem to work well with cool.io. This CI job is supposed to test ./run_benchmarks.rb, not TruffleRuby. Let's avoid failing the CI until TruffleRuby is fixed.

[eregon]

Could we skip the problematic benchmark instead?

I added it in #251 so it would at least notice regressions for existing benchmarks.
If we do step-level continue-on-error, nobody will notice any regression.

[eregon]

The error seems https://github.com/Shopify/yjit-bench/actions/runs/7064460115/job/19232535306 on fluentd.
I will take a look. Weird it regressed.

[eregon]

BTW I had forgotten to add it https://github.com/eregon/truffleruby-gem-tracker, I added it now, so I should notice within a week if it starts failing (after it's fixed).

[eregon]

Fix in truffleruby/truffleruby#3372

[k0kubun]

Thanks for fixing it. As of this moment, the truffleruby-head on GitHub Actions is still failing, so I added a way to skip only fluentd for truffleruby-head as you suggested 23b658e.

[eregon]

Thanks!

[eregon]

Self notes:

• Failure for liquid-c: https://github.com/Shopify/yjit-bench/actions/runs/7453361047/job/20278563105
  • RBASIC->klass in liquid-c
• Failure for lobsters: https://github.com/Shopify/yjit-bench/actions/runs/7453479468/job/20278936097
  • Incorrect header handling causing struct timespec to not be defined, probably similar to -std=c99 is disabling struct timespec socketry/io-event#88
  • old stringio using RBASIC->flags