**What does this PR do?**

This PR provides the final piece of the puzzle to fix #682: now that
ruby-dev-builder is building `3.4-asan` Rubies since
ruby/ruby-dev-builder#15, we can make them
available for use.

**Motivation:**

At Datadog, we're using ASan builds to check for issues
[in our library](https://github.com/datadog/dd-trace-rb),
and having a `3.4-asan` build improves the experience of having
these checks be a required CI step. Using the ASan builds built from
ruby-head means our CI could break because of unrelated issues/changes
from ruby-head.

**Additional Notes:**

`yarn` was trying really hard to update a bunch of things, which
generated A LOT of diff noise. I've manually pared them down just
to the actual changes related to adding the new variant so the
diff makes a bit more sense.

**How to test the change?**

I've already tested this in
https://github.com/DataDog/dd-trace-rb/actions/runs/13392865620/job/37696781848
and it seems to be working fine!