feat(ops/tvl-cache): add option for using experimental cache

Adds a `tvl.cache.builderball` option for cache *consumers* which enables the
experimental builderball cache. This cache uses whitby AND nevsky.

Change-Id: I4d99ba52d6ebd9a49e6fcb931d01e69383b15bcf
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12986
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

users/tazjin/nixos/tverskoy/default.nix

@@ -28,6 +28,7 @@ lib.fix (self: {
   ] ++ lib.optional (builtins.pathExists ./local-config.nix) ./local-config.nix;
 
   tvl.cache.enable = true;
+  tvl.cache.builderball = true;
 
   boot = {
     initrd.availableKernelModules = [ "nvme" "ehci_pci" "xhci_pci" "usb_storage" "sd_mod" "rtsx_pci_sdmmc" ];