refactor(tvix/store/tests/fixtures): use [u8; 20]

This makes the fixture more use-able when interacting with the trait,
the Bytes are only useful for the gRPC version.

Change-Id: Iaaea1adc6df18491f236a28c4343f5b4ee5fcfd3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11271
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Tested-by: BuildkiteCI

tvix/store/src/pathinfoservice/grpc.rs

@@ -195,7 +195,7 @@ mod tests {
         };
 
         let path_info = grpc_client
-            .get(fixtures::DUMMY_OUTPUT_HASH.to_vec().try_into().unwrap())
+            .get(fixtures::DUMMY_OUTPUT_HASH)
             .await
             .expect("must not be error");