build(chameleon): decrease minimum ruby version and test it

[c-dilks]

Stability of chameleon is rather critical, since it generates bindings (and eventually tests). Since users may have older versions of Ruby, we should be sure it works the same for those.

Unfortunately, installing an older version of Ruby is not so straightforward in the Arch container; we can't simply use a workflow action or the Arch Linux Archive. Instead, we use rbenv and cache the resulting build (analogous to a Python virtual environment).

[c-dilks]

@c-dilks deleted the branch main 4 minutes ago

I certainly did not! I think this is because I rebased onto main, then deleted the original base branch BASE-rcdb before trying to change it to main. This was a mistake, and seems to have confused GitHub.