Add lib64 to the rustc-link-search path

Additional context from #141 (comment)

I am actually not sure, but there was changed in cmake scripts in zlib-ng to 2.1. And the default cmake detection is a mess :D
So I think if we want to test it in CI, we have to also test against different distribution than Ubuntu or Debian.

From cmake source code:

```
 # Override this default 'lib' with 'lib64' iff:
 #  - we are on Linux system but NOT cross-compiling
 #  - we are NOT on debian
 #  - we are NOT building for conda
 #  - we are on a 64 bits system
```

Co-Authored-By: Jakub Onderka <ahoj@jakubonderka.cz>

build_zng.rs

@@ -22,10 +22,15 @@ pub fn build_zlib_ng(target: &str, compat: bool) {
 
     let includedir = install_dir.join("include");
     let libdir = install_dir.join("lib");
+    let libdir64 = install_dir.join("lib64");
     println!(
         "cargo:rustc-link-search=native={}",
         libdir.to_str().unwrap()
     );
+    println!(
+        "cargo:rustc-link-search=native={}",
+        libdir64.to_str().unwrap()
+    );
     let mut debug_suffix = "";
     let libname = if target.contains("windows") && target.contains("msvc") {
         if env::var("OPT_LEVEL").unwrap() == "0" {