nix: Refactor gh-actions and re-enable nightly builds (#31489)

Now that the nix build is working again, re-enable nightly builds and
refactor the workflow for re-use between nightly releases and CI jobs.

Release Notes:

- N/A

---------

Co-authored-by: Rahul Butani <rrbutani@users.noreply.github.com>

.github/workflows/release_nightly.yml

@@ -167,6 +167,10 @@ jobs:
       - name: Upload Zed Nightly
         run: script/upload-nightly linux-targz
 
+  bundle-nix:
+    needs: tests
+    uses: ./.github/workflows/nix.yml
+
   update-nightly-tag:
     name: Update nightly tag
     if: github.repository_owner == 'zed-industries'