chore: Clean up unnecessary spread

The customPrefs alias was introduced in mozilla#1039 as a direct alias,
but changed to a shallow copy in mozilla#2436 because the object was modified.
These changes have been dropped in mozilla#3136 but the swallow copy remained.

This patch completes the cleanup by reverting to a direct alias.

src/cmd/run.js

@@ -82,7 +82,7 @@ export default async function run(
 
   // Create an alias for --pref since it has been transformed into an
   // object containing one or more preferences.
-  const customPrefs = { ...pref };
+  const customPrefs = pref;
   const manifestData = await getValidatedManifest(sourceDir);
 
   const profileDir = firefoxProfile || chromiumProfile;