feat: update nix input to latest master

The nix c bindings have now merged into master, with some small changes since these bindings were created.
This commit updates the input, and adjusts it to the new API.

(cherry picked from commit aa02380bfcc6221615f65152de60cf4cca3bbbbe)

rust/nix-store/src/store.rs

@@ -83,7 +83,7 @@ impl Store {
             raw::nix_store_get_uri(
                 self.context.ptr(),
                 self.inner.ptr(),
-                callback_get_vec_u8 as *mut std::ffi::c_void,
+                Some(callback_get_vec_u8),
                 &mut raw_buffer as *mut Vec<u8> as *mut std::ffi::c_void,
             )
         };