ci(cachix): push only resulting packages without vendored deps

Free Cachix plan only has 5GiB of storage and vendored deps are more
than 1 GiB per platform. They are easier to build compared to
the server binaries which can take several hours to build,
so they can be removed.

flake.nix

@@ -39,13 +39,21 @@
       flake.lib = import ./lib { inherit nixpkgs; };
 
       perSystem =
-        { pkgs, ... }:
+        {
+          pkgs,
+          lib,
+          self',
+          ...
+        }:
         let
           craneLib = crane.mkLib pkgs;
         in
         {
           packages.server = pkgs.callPackage ./packages/server { inherit craneLib; };
           packages.frontends = pkgs.callPackage ./packages/frontends { };
+          githubActions = {
+            cachix.pathsToPush = lib.attrValues self'.packages;
+          };
         };
       githubActions = {
         cachix.enable = true;