gnupg: fix cross compilation

when `npth` upgraded 1.6 -> 1.7, the `npth-config` binary it previously
provided was removed, and functionality folded into `gpgrt-config`. see:
<https://github.com/gpg/npth/blob/417abd56fd7bf45cd4948414050615cb1ad59134/NEWS#L8>

pkgs/tools/security/gnupg/24.nix

@@ -50,7 +50,7 @@ stdenv.mkDerivation rec {
     "--with-libgcrypt-prefix=${libgcrypt.dev}"
     "--with-libassuan-prefix=${libassuan.dev}"
     "--with-ksba-prefix=${libksba.dev}"
-    "--with-npth-prefix=${npth}"
+    "GPGRT_CONFIG=${lib.getDev libgpg-error}/bin/gpgrt-config"
   ]
   ++ lib.optional guiSupport "--with-pinentry-pgm=${pinentry}/${pinentry.binaryPath or "bin/pinentry"}"
   ++ lib.optional withTpm2Tss "--with-tss=intel"