patch attic to prefetch 32 chunks instead of 2

overlay/attic/default.nix

@@ -0,0 +1,7 @@
+final: prev: {
+  attic-server = prev.attic-server.overrideAttrs {
+    patches = [
+      ./prefetch-32-chunks.patch
+    ];
+  };
+}

overlay/attic/prefetch-32-chunks.patch

@@ -0,0 +1,13 @@
+diff --git a/server/src/api/binary_cache.rs b/server/src/api/binary_cache.rs
+index 02e4857..71eeee8 100644
+--- a/server/src/api/binary_cache.rs
++++ b/server/src/api/binary_cache.rs
+@@ -262,7 +262,7 @@ async fn get_nar(
+ 
+         // TODO: Make num_prefetch configurable
+         // The ideal size depends on the average chunk size
+-        let merged = merge_chunks(chunks, streamer, storage, 2).map_err(|e| {
++        let merged = merge_chunks(chunks, streamer, storage, 32).map_err(|e| {
+             tracing::error!(%e, "Stream error");
+             e
+         });

overlay/default.nix

@@ -3,6 +3,7 @@ let
   overlays = [
     ./zipline
     ./bitwarden
+    ./attic
   ];
   importedOverlays = map (m: import m) overlays;
 in