feat(ops/modules): configure builderball cache setup

Configures an experimental setup for a builderball-based public cache.

This cache only includes the two build machines (whitby & nevsky), for the time
period where both of them exist simultaneously.

The idea is this:

All participating hosts run a harmonia binary cache locally (whitby already
does). They then run builderball instances pointing at each other's harmonia
caches (through dedicated public hostnames).

When a request comes in, the first matching cache address is returned and Nix
will substitute from there.

Change-Id: Ia7d5357fd5e04f77b460205544fa24e82b100230
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12975
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

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

@@ -8,7 +8,6 @@
   config = {
     services.nginx.virtualHosts."cache.tvl.su" = {
       serverName = "cache.tvl.su";
-      serverAliases = [ "cache.tvl.fyi" ];
       enableACME = true;
       forceSSL = true;