Convert changing actions to PRs

[diamondburned]

We could use PRs to ensure that actions requesting to add and update packages can be verified to still build without actually deploying them to production. It also simplifies the actions a lot.

Plan:

• Attempt to Nix build a Terraform plan. This ensures that we could fully validate that systems can be built.
  • Alternatively, do nix-build ./servers/*/ -A system. This saves us from having to work with
• Create an action for just building that, triggered by making or modifying a PR.
  • We don't reuse the deploy action because there's no easy way to share build outputs without uploading them to a server.
• Change existing add/update actions to create a PR instead.