declaration: handle nixpkgs edge case

ixx/src/action/index.rs

@@ -117,15 +117,19 @@ fn into_option(
 fn update_declaration(url_prefix: &Url, declaration: Declaration) -> anyhow::Result<Url> {
   let mut url = match declaration {
     Declaration::StorePath(path) => {
-      let idx = path
+      if path.starts_with("/") {
+        let idx = path
         .match_indices('/')
         .nth(3)
         .ok_or_else(|| anyhow!("Invalid store path: {}", path))?
         .0
         // +1 to also remove the / itself, when we join it with a url, the path in the url would
         // get removed if we won't remove it.
         + 1;
-      url_prefix.join(path.split_at(idx).1)?
+        url_prefix.join(path.split_at(idx).1)?
+      } else {
+        url_prefix.join(&path)?
+      }
     }
     Declaration::Url { name: _, url } => url,
   };