Migrate to build containers & start SteamRT4 builds

[Vauff]

Brings several benefits:

• Bumps clang version from 11 to 22, without having to reinstall each CI run
• AMBuild is installed in these containers, so we don't have to reinstall each CI run anymore
• Broadens Linux support to SteamRT4, which will be helpful to people who want to run that environment, or even people who try running the plugin on other unsupported distros that aren't compatible with SteamRT3 builds (Ubuntu 25.04 or Debian 13 cannot enable executable stack as shared object requires: Invalid argument #44, plugin stopped working #71)
• Reduces dependency on Valve's GitLab instance, which has seen temporary downtime in the past, whereas Actions & GHCR are both GitHub infrastructure