minor: fix image upload cleanup refs (#3228)

A small bug the robot caught while looking at #3222.

At the end of a successful image upload, we delete the temporary
snapshot and disk. If snapshot deletion succeeded but disk deletion
failed, the catch path called `cleanup()` while `snapshot.current` still
pointed at the already-deleted snapshot. Cleanup would then try to
delete that snapshot again, fail with 404, and never reach the remaining
disk cleanup.

Fix: Clear each ref immediately after its successful delete so later
cleanup only retries resources that still exist.

It would be nice to have a test for this but it would be pretty
elaborate for little gain.

Author: david-crespo

app/forms/image-upload.tsx

@@ -508,7 +508,9 @@ export default function ImageCreate() {
     // now delete the snapshot and the disk. don't use cleanup() because that
     // uses different mutations
     await deleteSnapshot.mutateAsync({ path: { snapshot: snapshot.current.id } })
+    snapshot.current = null
     await deleteDisk.mutateAsync({ path: { disk: disk.current.id } })
+    disk.current = null
 
     setAllDone(true)
   }