Add TruffleRuby in CI

[eregon]

Needs truffleruby/truffleruby#4159 and a dev build

[eregon]

@hsbt or @olleolleolle Could you merge this PR?
I think only you have permissions to merge on this repository (or at least both I and Stan don't have it).

[eregon]

truffleruby-head and not truffleruby because it needs changes from truffleruby/truffleruby#4159 which are not in a release yet.

[hsbt]

Merging is fine, but TruffleRuby is too unstable. CI gets stuck and fails every time something happens.

As you probably know, I'll remove it immediately if I see a failure. Please do your best to stabilize it first.

[eregon]

Thank you for merging.
This will help me remove the custom readline implementation in TruffleRuby (based on JLine) and just use the same code & logic as CRuby: truffleruby/truffleruby#3018

but TruffleRuby is too unstable. CI gets stuck and fails every time something happens.
As you probably know, I'll remove it immediately if I see a failure. Please do your best to stabilize it first.

I'm truly sorry about such cases.
Could you please include @eregon in the PR description when removing it so I know about it and can learn from such failures? (or whatever is most convenient for you, if e.g. a Slack message is more convenient due to pushing a commit directly to master or so, unfortunately @-mentions in commits no longer work).

I will also try to improve https://github.com/truffleruby/truffleruby-gem-tracker to detect cases of the truffleruby CI job being removed reliably (it's not trivial due to having to handle job cancellations, etc).

For instance with ruby/ostruct#68 I can't see the failure now because the log is no longer accessible, so I can't know what was the problem.
Maybe it was the transient concurrent Hash access within Psych/Bundler (truffleruby/truffleruby#3869), that was occuring in several CIs so I prioritized that and fixed it in 33.0.0.

I think TruffleRuby releases (ruby-version: truffleruby) are fairly stable overall, comparable to CRuby releases (ruby-version: ruby).
truffleruby-head is less stable than releases, similar to ruby-head, so I try to use truffleruby instead of truffleruby-head in CI whenever possible now for improving stability.

I'd also be OK to add continue-on-error at job-level but I think that doesn't help much as the job would still appear red in the GitHub UI (at least in the PR view and when looking at commit statuses). It would help if other CI jobs depend on it though, as they would still run then (but not a common case).
continue-on-error at step-level seems not a good option because it would hide all failures, including those caused by incompatible changes in the gem.

[eregon]

truffleruby-head is less stable than releases, similar to ruby-head, so I try to use truffleruby instead of truffleruby-head in CI whenever possible now for improving stability.

Regarding that, for CIs using uses: ruby/actions/.github/workflows/ruby_versions.yml@master, maybe we should only include truffleruby instead of truffleruby + truffleruby-head for improved stability in https://github.com/ruby/actions/blob/61edecb7338403daeea6ede2571892dba0cc0ffc/tool/update_ci_versions.rb#L23 ?
With the more frequent TruffleRuby releases I think that would be fine.