packaging: use structuredAttrs

staging-next banned !structuredAttrs && separateDebugInfo && disallowedRequisites
due to weird output interactions. Enable structuredAttrs so we can build again.

Also, fix type confusion that makes stdenv explode (https://github.com/NixOS/nixpkgs/issues/422989).

Co-authored-by: eldritch horrors <pennae@lix.systems>
Change-Id: Ic0c773394ee79e10d427f27750d59892d6d1f1d1
(cherry picked from commit 378b360bf8)

package.nix

@@ -186,6 +186,8 @@ assert (lintInsteadOfBuild -> lix-clang-tidy != null);
 stdenv.mkDerivation (finalAttrs: {
   inherit pname version;
 
+  __structuredAttrs = true;
+
   src = fileset.toSource {
     root = ./.;
     fileset = fileset.intersection baseFiles (