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/tests/fixtures.rs

@@ -109,7 +109,6 @@ lazy_static! {
         }),
         references: vec![DUMMY_OUTPUT_HASH.clone()],
         narinfo: None,
-        deriver: None,
     };
 
     /// A PathInfo message with .narinfo populated.
@@ -121,6 +120,7 @@ lazy_static! {
             nar_sha256: DUMMY_DIGEST.clone().into(),
             signatures: vec![],
             reference_names: vec![DUMMY_NAME.to_string()],
+            deriver: None,
         }),
       ..PATH_INFO_WITHOUT_NARINFO.clone()
     };