Don't always regenerate the exported array

This made launching external commands about 30% slower.

fish-rust/src/env/environment_impl.rs

@@ -584,7 +584,7 @@ fn export_array_needs_regeneration(&self) -> bool {
         }
 
         let mut cursor = self.export_array_generations.iter().fuse();
-        let mut mismatch = true;
+        let mut mismatch = false;
         self.enumerate_generations(|gen| {
             if cursor.next().cloned() != Some(gen) {
                 mismatch = true;