Compilation of vendored OpenSSL inside Nix is broken since
<https://github.com/alexcrichton/openssl-src-rs/pull/229> due to build
script changes.
There is anyway no need to compile vendored OpenSSL as nixpkgs already
contains OpenSSL package.
This fixes `nix build .#deltachat-rpc-server-x86_64-linux` and similar
commands which are used during releases.