Reduce HTTP requests for checksum file in fetch-checksum

[eleanorLYJ]

After adding more test cases, the CI pipeline fails due to excessive HTTP requests to GitHub during the fetch-checksum step. This triggers rate-limiting errors (403: rate limit exceeded).

Proposed Solution:

Check if the checksum file exists and is less than one day old before fetching it. This will reduce unnecessary requests while ensuring the file is up-to-date.

[jserv]

Network quality issues may sometimes cause unexpected disconnections when downloading from certain remote sites, requiring retries.