openssh: don't let configure override SSH_KEYSIGN

While 9fe10288f0 ensured that the
ssh-keysign path is searched for in PATH if not absolute,
it doesn't prevent the configure script from defaulting to an
absolute path in $out/libexec, making the whole effort rather
pointless.

pkgs/tools/networking/openssh/default.nix

@@ -88,6 +88,8 @@ stdenv.mkDerivation rec {
     ++ optional stdenv.isDarwin "--disable-libutil"
     ++ optional (!linkOpenssl) "--without-openssl";
 
+  buildFlags = [ "SSH_KEYSIGN=ssh-keysign" ];
+
   enableParallelBuilding = true;
 
   hardeningEnable = [ "pie" ];