Strip RUST_BACKTTRACE and RUST_LIB_BACKTRACE before invoking buck2

Summary:
Buck has a [known issue](https://github.com/facebook/buck2/blob/71da19b58242cb6ad9fd4b3d126ed6d20ae659bc/app/buck2_interpreter_for_build/src/attrs/coerce/attr_type/source.rs#L48) where the daemon becomes extremely slow if invoked with the `RUST_BACKTRACE` env variable set.

In ELP, we set the RUST_BACKTRACE variable while configuring logging. Later on, we invoke Buck during project loading.

While there's no evidence of Buck2 builds being particularly slow, we can be on the safe side and apply the same patch that Rust Analyzer applied, stripping those two environment variables before the Buck2 invocation.

Reviewed By: davidbarsky

Differential Revision: D61392415

fbshipit-source-id: 177e9ec79ef54f0bd3bf3dfe2a186e803a7b8589

crates/project_model/src/buck.rs

@@ -91,6 +91,10 @@ impl BuckConfig {
         static BUCK_GLOBAL_LOCK: Mutex<()> = Mutex::new(());
         let guard = BUCK_GLOBAL_LOCK.lock();
         let mut cmd = Command::new("buck2");
+        // buck2 doesn't handle the RUST_BACKTRACE option well and can become extremely
+        // slow. Until that is fixed ensure the option is not propagated to buck2.
+        cmd.env_remove("RUST_BACKTRACE")
+            .env_remove("RUST_LIB_BACKTRACE");
         cmd.arg("--isolation-dir");
         cmd.arg("lsp");
         cmd.current_dir(self.buck_root());