dnscrypt-wrapper: disable fortify3 hardening flag

This was enabled by default in db3e94c3 and breaks the key generation,
specifically `dnscrypt-wrapper --gen-crypt-keypair` fails with:

Generate crypt key pair... ok.
Secret key stored in 2.dnscrypt-cert.server.key
*** buffer overflow detected ***: terminated

FAQ:

1. Is the buffer overflow real? Probably.

2. Is it maintained? Is upstream going to fix it? Not really.

3. Are you willing to investigate and patch it yourself? Nope.

pkgs/tools/networking/dnscrypt-wrapper/default.nix

···
 
   enableParallelBuilding = true;
 
+  # causes `dnscrypt-wrapper --gen-provider-keypair` to crash
+  hardeningDisable = [ "fortify3" ];
+
   nativeBuildInputs = [ pkg-config autoreconfHook ];
   buildInputs = [ libsodium libevent ];