build: do not vendor OpenSSL when cross-compiling

[link2xt]

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.