ci(gh-actions): Improvement to the CI pipeline

[PetarKirov]

• ci(gh-actions): Enable CI workflow on workflow_dispatch (manual trigger) and merge queue
• ci(gh-actions): Use repo-wide variable for the cachix-action name input
• ci(gh-actions): Replace the Cachix install Nix action with Determinate Systems' action
• ci(gh-actions): Generate CI matrix automatically
• ci(pkgs/ldc): Allow failure on ["x86_64-darwin" "aarch64-darwin"]
• ci(gh-actions): Define "results" job that depends on the result of the whole matrix