EROFS testfile not found

### What happened in your environment?

github CI

### What did you expect to happen?

2025/09/30 02:53:58.942229|ERROR|th=00005581EBDD0810|trace_test.cpp:76|download_erofs_img:download failed: https://github.com/salvete/erofs-imgs/raw/main/alpine.img errno=2(No such file or directory)

### How can we reproduce it?

github CI

### What is the version of your Overlaybd?

latest

### What is your OS environment?

all

### Are you willing to submit PRs to fix it?

- [ ] Yes, I am willing to fix it.