Pass substituters in so local builds aren't trying to hit the in-kubernetes cache.

.webhook_bridge/pipeline-build-homepage-staging.yaml

@@ -142,6 +142,8 @@ spec:
               - "type=registry,ref=$(params.image-name):buildcache,mode=max,compression=zstd,compression-level=22,rewrite-timestamp=true,image-manifest=true,oci-mediatypes=true"
               - --opt
               - build-arg:SOURCE_DATE_EPOCH=$(tasks.get-git-commit-time.results.unix-time)
+              - --opt
+              - "build-arg:NIX_SUBSTITUTERS=http://ncps.nix-pull-through-cache.svc.cluster.local:80 https://cache.nixos.org"
           - name: BUILDKITD_TOML
             value: |
               debug = true