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/ci.yml

@@ -714,6 +714,7 @@ jobs:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 
   nix-build:
+    name: Build with Nix
     uses: ./.github/workflows/nix.yml
     if: github.repository_owner == 'zed-industries' && contains(github.event.pull_request.labels.*.name, 'run-nix')
     with: