feat: add hydra jobs to build all packages

nix/default.nix

@@ -4,5 +4,7 @@
     ./packages
     ./shells
     ./tests
+
+    ./hydra.nix
   ];
 }

nix/hydra.nix

@@ -0,0 +1,9 @@
+{ inputs, ... }:
+let
+  lib = inputs.nixpkgs.lib;
+in
+{
+  flake.hydraJobs.packages = builtins.mapAttrs (
+    _name: lib.hydraJob
+  ) inputs.self.packages.x86_64-linux;
+}