chore: add nix checks

Signed-off-by: Sefa Eyeoglu <[e-mail]>

flake.nix

···
       imports = [
         inputs.pre-commit-hooks.flakeModule
 
+        ./nix/checks.nix
         ./nix/dev.nix
         ./nix/packages.nix
       ];

nix/checks.nix

···
+{lib, ...}: {
+  perSystem = {config, ...}: {
+    checks = lib.mapAttrs' (n: lib.nameValuePair "package-${n}") config.packages;
+  };
+}