Define BuildKite pipelines in Nix

After a handful of failed attempts to run lint-secrets.sh due to a missing
`git-secrets` executable on my git server, I decided that now was a good time to
use Nix to define my BuildKite pipelines.

TL;DR:
- Delete ci/scripts directory
- Define ci/pipelines/{briefcase,socrates}.nix

Outside of this repository:
- I logged into my admin account at git.wpcarro.dev and changed my Gitea
  post-receive hook to trigger the briefcase pipeline
- I logged into my BuildKite account, deleted my build-briefcase pipeline,
  created a new briefcase pipeline that called:

  ```shell
  nix-build -A ci.pipelines.briefcase -o briefcase.yaml
  buildkite-agent pipeline upload briefcase.yaml
  ```

One day I will audit all of my ad-hoc, non-mono-repo activity (like the steps I
listed above) and attempt to fit everything herein... one step at a time,
though!

ci/pipelines/socrates.nix

@@ -0,0 +1,18 @@
+{ pkgs, ... }:
+
+let
+  pipeline.steps = [
+    {
+      command = ''
+        nix-build '<nixpkgs/nixos>' \
+          -I briefcase="$(pwd)" \
+          -I nixpkgs=/var/lib/buildkite-agent-socrates/nixpkgs-channels \
+          -I nixos-config=nixos/socrates/default.nix \
+          -A system \
+          --no-out-link \
+          --show-trace
+      '';
+      label = ":laptop: Socrates [build]";
+    }
+  ];
+in pkgs.writeText "socrates.yml" (builtins.toJSON pipeline)