Add dependency 'ar' and fix path

pkgs/by-name/de/debsigs/package.nix

···
 {
+  binutils,
   lib,
   perlPackages,
   fetchFromGitLab,
···
   };
 
   sourceRoot = "${src.name}/perl";
+
+  postPatch = ''
+    substituteInPlace arf.pm \
+      --replace-fail /usr/bin/ar ${binutils.bintools}/bin/ar
+  '';
 
   meta = {
     description = "Manipulate the cryptographic signatures stored inside a .deb file";