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>

flake.nix

@@ -54,11 +54,9 @@
     };
 
   nixConfig = {
-    extra-substituters = [
-      "https://zed-industries.cachix.org"
-    ];
+    extra-substituters = [ "https://zed.cachix.org" ];
     extra-trusted-public-keys = [
-      "zed-industries.cachix.org-1:QW3RoXK0Lm4ycmU5/3bmYRd3MLf4RbTGPqRulGlX5W0="
+      "zed.cachix.org-1:/pHQ6dpMsAZk2DiP4WCL0p9YDNKWj2Q5FL20bNmw1cU="
     ];
   };
 }