refactor(nix-compat/wire/bytes/reader): use fn for PhantomData

We don't actually hold a Tag, we just want to bind the type.

Change-Id: Ida67c026f852ed54c3f18df914cf5c31e6227fd5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11541
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/nix-compat/src/wire/bytes/reader/trailer.rs

@@ -58,7 +58,7 @@ pub(crate) struct ReadTrailer<R, T: Tag = Pad> {
     data_len: u8,
     filled: u8,
     buf: T::Buf,
-    _phantom: PhantomData<*const T>,
+    _phantom: PhantomData<fn(T) -> T>,
 }
 
 /// read_trailer returns a [Future] that reads a trailer with a given [Tag] from `reader`