xdg-utils: enable cross compilation

native build produces binaries with `#!${bash}/bin/sh` as shebang.
seems to be a consequence of nixpkgs using bash in the builder, and
`patchShebangs` sees that. OTOH cross builds would get `#!/bin/sh` because
the builder's bash isn't eligible as a runtime shebang. i can't say
which is most "correct" in the context of this tool, but shipping bash
in `buildInputs` where `patchShebangs` can see it at least gets us *consistency*.

pkgs/tools/X11/xdg-utils/default.nix

···
 , file, libxslt, docbook_xml_dtd_412, docbook_xsl, xmlto
 , w3m, gnugrep, gnused, coreutils, xset, perlPackages
 , mimiSupport ? false, gawk
+, bash
 , glib
 , withXdgOpenUsePortalPatch ? true }:
 
···
   # just needed when built from git
   nativeBuildInputs = [ libxslt docbook_xml_dtd_412 docbook_xsl xmlto w3m ];
 
+  # explicitly provide a runtime shell so patchShebangs is consistent across build platforms
+  buildInputs = [ bash ];
+
   postInstall = lib.optionalString mimiSupport ''
     cp ${mimisrc}/xdg-open $out/bin/xdg-open
   '' + ''
···
     license = if mimiSupport then licenses.gpl2 else licenses.free;
     maintainers = [ maintainers.eelco ];
     platforms = platforms.all;
-    broken = !(stdenv.buildPlatform.canExecute stdenv.hostPlatform);
   };
 }