feat(tvl-kit): Expose lazy-deps

This adds the //nix/lazy-deps tool at //lazy-deps in tvl-kit.

A CI step is added for the kit that uses this to lazily build an
example tool (magrathea).

Change-Id: Ibd6d69c83b87bd6e0766942d73297621f2593113
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5514
Tested-by: BuildkiteCI
Reviewed-by: ezemtsov <eugene.zemtsov@gmail.com>

views/kit/buildkite.yml

@@ -6,5 +6,11 @@
 steps:
   - command: "nix-build --no-out-link -A besadii"
     label: ":nix: besadii"
+
   - command: "nix-build --no-out-link -A magrathea"
     label: ":nix: magrathea"
+
+  - label: ":nix: lazy-deps"
+    command: |
+      nix-build -E 'with import ./. {}; lazy-deps { mg.attr = "magrathea"; }'
+      result/bin/mg