ci: use Arch Linux container on Linux runners

[c-dilks]

ubuntu-latest GitHub runners have packages which are quite out of date, which requires us to build newer versions fmt and yaml-cpp in separate CI jobs. If the list of dependencies grows, this is not practical; nor is it practical for users.

So let's use a Docker container which provides easy access to up-to-date dependency versions.

Arch is chosen over Alma 9; see comments below.

This could be the basis for creating our own image for iguana users.

[c-dilks]

According to pkgs.org, Alma 9 also lacks iguana dependencies:

• yaml-cpp is even older than ubuntu-latest GitHub runner's: https://pkgs.org/search/?q=yaml-cpp
• fmt is not available in Alma repos, but v8 is in EPEL: https://pkgs.org/search/?q=fmt

I was afraid this would be the case with enterprise distros, where many packages are often several major versions behind.

So for now, let's switch to Arch where everything is guaranteed to be up-to-date, since our current priority is working CI pipelines for #91.

[c-dilks]

archlinux/archlinux:latest has

extra/fmt       10.2.0-1       1.58 MiB       0.36 MiB
extra/yaml-cpp  0.8.0-1        0.42 MiB       0.13 MiB

[c-dilks]

Image archlinux with fmt and yaml-cpp is around 150MB. We could cache it, but it may be easier to just install dependencies on each job.

ROOT will be heavy, but perhaps for that we may use a cvmfs action; ROOT also supplies Arch images.

[c-dilks]

On the other hand, Arch forces us to install all the dependencies...