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

.github/workflows/nix.yml

@@ -56,6 +56,7 @@ jobs:
           name: zed
           authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
           pushFilter: "${{ inputs.cachix-filter }}"
+          cachixArgs: '-v'
 
       - run: nix build .#${{ inputs.flake-output }} -L --accept-flake-config