libadwaita: passthru outputBin as `bin`

pkgs/by-name/li/libadwaita/package.nix

···
     "devdoc"
   ];
   outputBin = "devdoc"; # demo app
+  passthru.bin = finalAttrs.finalPackage.${finalAttrs.outputBin}; # fixes lib.getExe
 
   src = fetchFromGitLab {
     domain = "gitlab.gnome.org";