feat(whitby): switch from nix-serve to harmonia for the cache

Harmonia is, ostensibly, faster and better and, most importantly, not a giant
pile of wonky Perl.

I've tested locally that Harmonia works with Nix 2.3 (on both ends), so I think
we should be good to go here.

We have a vendored copy of the upstream module for now. We need to fix Nix 2.3
compatibility in upstream for the module, but the service itself works fine.

Change-Id: I3897bb02b83bd466b6fe7077c05728ac49ea4406
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12517
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>
Reviewed-by: sterni <sternenseemann@systemli.org>

ops/modules/www/cache.tvl.su.nix

@@ -17,13 +17,8 @@
           alias /run/agenix/nix-cache-pub;
         }
 
-        location = /nix-cache-info {
-          add_header Content-Type text/plain;
-          return 200 "StoreDir: /nix/store\nWantMassQuery: 1\nPriority: 50\n";
-        }
-
         location / {
-          proxy_pass http://localhost:${toString config.services.nix-serve.port};
+          proxy_pass http://${config.services.depot.harmonia.settings.bind};
         }
       '';
     };