nix: Add job names and garnix substitutor (#31625)

This should result in some additional cache hits as I personally use
garnix.

Also added `-v` cachix arg to try to figure out why CI jobs aren't
pushing any paths. Right now they just show ["Pushing is
disabled."](https://github.com/zed-industries/zed/actions/runs/15293723678/job/43018512167#step:13:3)
but I'm not sure if that's due to the `pushFilter` or misconfigured
secrets.

Release Notes:

- N/A

flake.nix

@@ -54,9 +54,13 @@
     };
 
   nixConfig = {
-    extra-substituters = [ "https://zed.cachix.org" ];
+    extra-substituters = [
+      "https://zed.cachix.org"
+      "https://cache.garnix.io"
+    ];
     extra-trusted-public-keys = [
       "zed.cachix.org-1:/pHQ6dpMsAZk2DiP4WCL0p9YDNKWj2Q5FL20bNmw1cU="
+      "cache.garnix.io:CTFPyKSLcx5RMJKfLo5EEPUObbA78b0YQ2DTCJXqr9g="
     ];
   };
 }