calibre: fix build of desktop items

Broken by https://github.com/NixOS/nixpkgs/pull/75729 since the desktop
items are only valid after post-processing. There's probably multiple
better ways to do this, but I'm not a calibre maintainer and I just want
to get this unbroken as quickly as possible.

pkgs/applications/misc/calibre/default.nix

@@ -113,6 +113,7 @@ mkDerivation rec {
   disallowedReferences = [ podofo.dev ];
 
   calibreDesktopItem = makeDesktopItem {
+    fileValidation = false; # fails before substitution
     name = "calibre-gui";
     desktopName = "calibre";
     exec = "@out@/bin/calibre --detach %F";
@@ -159,6 +160,7 @@ mkDerivation rec {
   };
 
   ebookEditDesktopItem = makeDesktopItem {
+    fileValidation = false; # fails before substitution
     name = "calibre-edit-book";
     desktopName = "Edit E-book";
     genericName = "E-book Editor";
@@ -171,6 +173,7 @@ mkDerivation rec {
   };
 
   ebookViewerDesktopItem = makeDesktopItem {
+    fileValidation = false; # fails before substitution
     name = "calibre-ebook-viewer";
     desktopName = "E-book Viewer";
     genericName = "E-book Viewer";