ci performance: add check to ensure nothing depends on the whole repo

Since this project is an ever growing monorepo, having derivations depending on the whole repo leads to bad CI performance, as the cache is busted on every commit.

-> We never want any derivations depend on the whole repo

...except: the test that tests that nothing depends on the whole repo, which is added by this commit.

For now only add this check to packages to allow contributors to build it locally.
We might want to add it to the CI later once all occurrences are fixed.

checks/flake-module.nix

@@ -14,6 +14,7 @@ in
     ./installation/flake-module.nix
     ./morph/flake-module.nix
     ./nixos-documentation/flake-module.nix
+    ./sanity-checks/dont-depend-on-repo-root.nix
   ];
   perSystem =
     {
@@ -54,11 +55,17 @@ in
             syncthing = import ./syncthing nixosTestArgs;
           };
 
+          packagesToBuild = lib.removeAttrs self'.packages [
+            # exclude the check that checks that nothing depends on the repo root
+            # We might want to include this later once everything is fixed
+            "dont-depend-on-repo-root"
+          ];
+
           flakeOutputs =
             lib.mapAttrs' (
               name: config: lib.nameValuePair "nixos-${name}" config.config.system.build.toplevel
             ) (lib.filterAttrs (n: _: !lib.hasPrefix "test-" n) self.nixosConfigurations)
-            // lib.mapAttrs' (n: lib.nameValuePair "package-${n}") self'.packages
+            // lib.mapAttrs' (n: lib.nameValuePair "package-${n}") packagesToBuild
             // lib.mapAttrs' (n: lib.nameValuePair "devShell-${n}") self'.devShells
             // lib.mapAttrs' (name: config: lib.nameValuePair "home-manager-${name}" config.activation-script) (
               self'.legacyPackages.homeConfigurations or { }