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>

third_party/overlays/tvl.nix

@@ -79,10 +79,6 @@ depot.nix.readTree.drvTargets {
     };
   });
 
-  # nix-serve does not work with nix 2.4
-  # https://github.com/edolstra/nix-serve/issues/28
-  nix-serve = super.nix-serve.override { nix = self.nix_2_3; };
-
   # Avoid builds of mkShell derivations in CI.
   mkShell = super.lib.makeOverridable (args: (super.mkShell args).overrideAttrs (_: {
     passthru = {