Stabilize joinir tests and env guards

src/runner/json_v0_bridge/lowering.rs

@@ -599,6 +599,7 @@ mod tests {
     use super::*;
     use std::fs;
     use std::path::PathBuf;
+    use std::sync::{Mutex, OnceLock};
 
     fn temp_path(name: &str) -> PathBuf {
         let mut p = std::env::temp_dir();
@@ -606,12 +607,18 @@ mod tests {
         p
     }
 
+    fn env_guard() -> &'static Mutex<()> {
+        static GUARD: OnceLock<Mutex<()>> = OnceLock::new();
+        GUARD.get_or_init(|| Mutex::new(()))
+    }
+
     fn dummy_module() -> MirModule {
         MirModule::new("test-module".to_string())
     }
 
     #[test]
     fn writes_file_when_dump_path_is_set() {
+        let _lock = env_guard().lock().unwrap();
         let path = temp_path("mir_dump_path");
         let _ = fs::remove_file(&path);
         std::env::set_var("RUST_MIR_DUMP_PATH", path.to_string_lossy().to_string());
@@ -634,6 +641,7 @@ mod tests {
 
     #[test]
     fn does_not_write_when_dump_path_is_unset() {
+        let _lock = env_guard().lock().unwrap();
         std::env::remove_var("RUST_MIR_DUMP_PATH");
         let path = temp_path("mir_dump_path_unset");
         let _ = fs::remove_file(&path);