Disable clippy nightly warning about const TLS

This complains:

> warning: initializer for `thread_local` value can be made `const`

But doing the suggested fix breaks the build on our MSRV, with:

> warning: a `const` item should never be interior mutable

So we can't really win.

src/lib.rs

@@ -54,6 +54,7 @@ include!(concat!(env!("OUT_DIR"), "/version.rs"));
 // Rust code, we model these thread locals as a stack, so we can always
 // restore the previous version.
 thread_local! {
+    #[allow(clippy::thread_local_initializer_can_be_made_const)]
     pub(crate) static USERDATA: RefCell<Vec<Userdata>> = RefCell::new(Vec::new());
 }