Cross-compilation under Nix is broken since #229

[link2xt]

229 changed build scripts in a way that breaks builds of vendored OpenSSL under Nix.

On Linux I worked around by switching to statically built OpenSSL from nixpkgs: https://github.com/deltachat/deltachat-core-rust/pull/5316

But for Windows platform this did not work.

Log failure when trying to build vendored OpenSSL for Windows:

2024-03-04T21:38:46.8132887Z        > error: failed to run custom build command for `openssl-sys v0.9.101`
2024-03-04T21:38:46.8133558Z        >
2024-03-04T21:38:46.8133860Z        > Caused by:
2024-03-04T21:38:46.8135115Z        >   process didn't exit successfully: `/build/dummy-src/target/release/build/openssl-sys-60fc2e3a843f5bde/build-script-main` (exit status: 101)
2024-03-04T21:38:46.8136334Z        >   --- stdout
2024-03-04T21:38:46.8136991Z        >   cargo:rerun-if-env-changed=X86_64_PC_WINDOWS_GNU_OPENSSL_NO_VENDOR
2024-03-04T21:38:46.8137750Z        >   X86_64_PC_WINDOWS_GNU_OPENSSL_NO_VENDOR unset
2024-03-04T21:38:46.8138433Z        >   cargo:rerun-if-env-changed=OPENSSL_NO_VENDOR
2024-03-04T21:38:46.8139002Z        >   OPENSSL_NO_VENDOR unset
2024-03-04T21:38:46.8139428Z        >
2024-03-04T21:38:46.8139760Z        >   --- stderr
2024-03-04T21:38:46.8141194Z        >   thread 'main' panicked at /nix/store/nqxnkxrqi9zx3zd60slbvg6pnyiprysv-crates-io-dependencies/openssl-src-300.2.3+3.2.1-5cff92b6f71555b61bb9315f7c64da3ca43d87531622120fea0195fc761b4843/src/lib.rs:648:13:
2024-03-04T21:38:46.8143424Z        >   can't copy symlink "/nix/store/nqxnkxrqi9zx3zd60slbvg6pnyiprysv-crates-io-dependencies/openssl-src-300.2.3+3.2.1-5cff92b6f71555b61bb9315f7c64da3ca43d87531622120fea0195fc761b4843/openssl/README.md"
2024-03-04T21:38:46.8144799Z        >   note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
2024-03-04T21:38:46.8145448Z        > warning: build failed, waiting for other jobs to finish...
2024-03-04T21:38:46.8145980Z        > [naersk] cargo returned with exit code 101, exiting