chore(ops/pipelines): pin anchoring step to whitby agents

Change-Id: Ia7d556667faf44094cb5545e4f38d85260c5aafa
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12989
Reviewed-by: sterni <sternenseemann@systemli.org>
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

ops/pipelines/depot.nix

@@ -27,6 +27,11 @@ let
         command = ''
           nix-build -A ci.gcroot --out-link /nix/var/nix/gcroots/depot/canon
         '';
+
+        # Ensure that anchoring happens on whitby, so that cache.tvl.su always
+        # has the full cache. Unanchored machines may garbage collect live
+        # paths.
+        agents.hostname = "whitby"; # TODO(tazjin): b/433
       }
     ];
   };