add GHA

[zimbatm]

This PR configures the repo with GHA, a build matrix and Cachix.

For Cachix pushes to work, you'll have to create your own cache, set the
CACHIX_AUTH_TOKEN in the repo secrets and update the name of the cache from
"tmp-snowflake" (see the last commit).

Hope you enjoy the work, and let me know if you have any questions!

• ci: add GitHub Actions matrix based on flake checks
• flake: disable broken lga-test1.tenant-29c7a3-baggie.coreweave.cloud
• flake: add all packages to checks
• flake: add nixos configurations to the checks
• flake: disable broken home config
• flake: add x86_64-darwin to the list of supported systems
• flake: add home configurations to the checks
• ci: fixup
• ci: fix build matrix attributes
• ci: make the build matrix more readable
• ci: only build packages on supported systems
• wayland-push-to-talk-fix: only compile on linux
• ci: disable fail-fast
• ci: make space for large NixOS closures
• ci: cache build results

[anthr76]

That's great! Thanks very much @zimbatm :)

[anthr76]

I actually already have a auth token wired. Just need to update the cache name