PR checks now also run tests on openSUSE, Nix, Azure Linux containers

[reubeno]

Hello brush folks! We recently expanded PR checks to run the standard brush test suite within 3 additional environments:

• openSUSE Tumbleweed container: opensuse/tumbleweed:latest
• Nix container: nixos/nix:latest
• Azure Linux 3.0 container: mcr.microsoft.com/azurelinux/base/core:3.0

This was a natural continuation of the work referenced in #243, and in the case of NixOS, also spurred on by the side discussion (#395) of issues running the test suite on NixOS.

If you encounter any issues with these tests or have any feedback on this, please do respond here or file new issues. If there are other distros you'd like to see added, then please feel free to respond here or, better yet, create a PR and reference this discussion.