Fix: Ignore CARGO_BUILD_TARGET in tests

When build target triples are explicitly specified, the layout of the
target directory is changed from `target/...` to `target/<triple>/...`.

See: https://doc.rust-lang.org/cargo/guide/build-cache.html

This caused bin tests failures in `tests/profile.rs`. This patch
proposes that we simply ignore the `CARGO_BUILD_TARGET` env var in
tests to avoid such problems.

A new test is introduced to ensure the correct behavior.

tests/profile.rs

@@ -45,6 +45,9 @@ fn build_using_bin(extra_args: &[&str]) -> tempfile::TempDir {
     Command::new(cbindgen_path)
         .current_dir(expand_dep_test_dir)
         .env("CARGO_EXPAND_TARGET_DIR", tmp_dir.path())
+        .env_remove("CARGO_BUILD_TARGET")
+        // ^ avoid unexpected change of layout of the target directory;
+        // ... see: https://doc.rust-lang.org/cargo/guide/build-cache.html
         .args(extra_args)
         .output()
         .expect("build should succeed");
@@ -87,6 +90,18 @@ fn bin_default_uses_debug_build() {
     assert_eq!(get_contents_of_dir(target_dir.path()), &["debug"]);
 }
 
+#[test]
+fn bin_ignore_cargo_build_target_in_tests() {
+    use std::env;
+    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();
+}
+
 #[test]
 fn bin_explicit_debug_build() {
     let target_dir = build_using_bin(&["--profile", "debug"]);