Build for Ubuntu 20.04 instead of 22.04

[Primebrook]

Issue

We're precompiling for ubuntu-22.04 which comes with glibc 2.35. Our CI and Prod builds use Debian for all of our Elixir apps and the Debian version they use comes with glibc 2.33.

12:07:53.333 [warning] The on_load function for module Elixir.Exstatic.Native returned:
{:error,
 {:load_failed,
  ~c"Failed to load NIF library: '/lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.33' not found (required by /root/project/_build/test/lib/exstatic/priv/native/libexstatic-v0.2.0-nif-2.16-x86_64-unknown-linux-gnu.so)'"}}

Also I added a couple of extra build targets.

This PR

Switches Linux builds to ubuntu-20.04 which comes with glibc 2.31. Github Actions doesn't come with Debian runners 🫠, so this is the next best thing (if this works).

glibc 2.33 should be compatible with 2.31.