refactor(tvix/store): move deriver field into narinfo submessage

This information is present in the .narinfo files, it should have gone
there.

Change-Id: Ib43d0cf30c2795bf1fe77c46646174353ade0458
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9794
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Tested-by: BuildkiteCI

tvix/store/src/bin/tvix-store.rs

@@ -302,8 +302,8 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
                                 nar_sha256: nar_sha256.to_vec().into(),
                                 signatures: vec![],
                                 reference_names: vec![],
+                                deriver: None,
                             }),
-                            deriver: None,
                         };
 
                         // put into [PathInfoService], and return the PathInfo that we get back