Fix: Ignore CARGO_BUILD_TARGET in lib tests too

This follows from the previous commit. It turns out that
`std::env::set_var` is unsafe: the `set_var` in bin tests leaked
through the threads and caused `lib` tests to fail.

To mitigate this problem we simply remove the `CARGO_BUILD_TARGET`
variable in lib tests as well. This also improves correctness.
Meanwhile we put `std::env::set_var` and `remove_var` in unsafe blocks
as it would be required by Rust 2024.

tests/profile.rs

@@ -1,6 +1,7 @@
 use cbindgen::*;
 
 use serial_test::serial;
+use std::env;
 use std::path::{Path, PathBuf};
 use std::process::Command;
 
@@ -17,7 +18,12 @@ fn build_using_lib(config: fn(Builder) -> Builder) -> tempfile::TempDir {
         .tempdir()
         .expect("Creating tmp dir failed");
 
-    std::env::set_var("CARGO_EXPAND_TARGET_DIR", tmp_dir.path());
+    unsafe {
+        env::set_var("CARGO_EXPAND_TARGET_DIR", tmp_dir.path());
+        env::remove_var("CARGO_BUILD_TARGET");
+        // ^ avoid unexpected change of layout of the target directory;
+        // ... see: https://doc.rust-lang.org/cargo/guide/build-cache.html
+    }
     let builder = Builder::new()
         .with_config(Config::from_file(expand_dep_test_dir.join("cbindgen.toml")).unwrap())
         .with_crate(expand_dep_test_dir);
@@ -92,14 +98,16 @@ fn bin_default_uses_debug_build() {
 
 #[test]
 fn bin_ignore_cargo_build_target_in_tests() {
-    use std::env;
-    env::set_var("CARGO_BUILD_TARGET", "x86_64-unknown-linux-gnu");
+    unsafe {
+        env::set_var("CARGO_BUILD_TARGET", "x86_64-unknown-linux-gnu");
+    }
     assert_eq!(
         env::var("CARGO_BUILD_TARGET"),
         Ok("x86_64-unknown-linux-gnu".into())
     );
     // ^ this env var should be ignored:
     bin_default_uses_debug_build();
+    lib_default_uses_debug_build();
 }
 
 #[test]