fix(oxlint): gate custom allocators by feature flag (#3945)

This tweaks the conditional compilation of custom allocators by wiring
the dedicated Cargo feature flag `allocator` to the source code. It
ensures that allocator gating is directly checking the relevant feature
instead of trying to infer it from the build profile.

apps/oxlint/src/main.rs

@@ -1,11 +1,11 @@
 #![cfg(not(miri))] // Miri does not support custom allocators
 
-#[cfg(not(debug_assertions))]
+#[cfg(feature = "allocator")]
 #[cfg(not(target_env = "msvc"))]
 #[global_allocator]
 static GLOBAL: jemallocator::Jemalloc = jemallocator::Jemalloc;
 
-#[cfg(not(debug_assertions))]
+#[cfg(feature = "allocator")]
 #[cfg(target_os = "windows")]
 #[global_allocator]
 static GLOBAL: mimalloc::MiMalloc = mimalloc::MiMalloc;