chore: fix CI to make all checks green

- define a nix default package
- try to fix CI
- fix/improve (?) CI even more (??)

nix/packages/default.nix

@@ -6,4 +6,10 @@
     ./rust.nix
     ./uwulib
   ];
+
+  perSystem =
+    { self', ... }:
+    {
+      packages.default = self'.packages.continuwuity-default-bin;
+    };
 }

nix/packages/rust.nix

@@ -3,6 +3,7 @@
   perSystem =
     {
       system,
+      lib,
       ...
     }:
     {