feat(ops/modules): Add shared module for TVL cache

Add a shared nixos module for configuring whitby as a binary nix cache,
and refactor tverskoy to use this module.

This is enabled via an option to pave the way for including it as an
import in all depot-generated nixos configs at some point in the future.

Change-Id: I6dcc0e8eb48b1ac34457666dceebeedd5da6c526
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4344
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: wpcarro <wpcarro@gmail.com>
Autosubmit: grfn <grfn@gws.fyi>

ops/modules/tvl-cache.nix

@@ -0,0 +1,19 @@
+{ config, lib, pkgs, ... }:
+
+{
+  options = {
+    tvl.cache.enable = lib.mkEnableOption "the TVL binary cache";
+  };
+
+  config = lib.mkIf config.tvl.cache.enable {
+    nix = {
+      binaryCachePublicKeys = [
+        "cache.tvl.su:kjc6KOMupXc1vHVufJUoDUYeLzbwSr9abcAKdn/U1Jk="
+      ];
+
+      binaryCaches = [
+        "https://cache.tvl.su"
+      ];
+    };
+  };
+}