Fix secondary rate limit when downloading artifacts in slack report (#46796)
Use forked download-artifact with chunk-delay to avoid rate limits
Switch to ydshieh/download-artifact which adds a `chunk-delay` input,
inserting a pause between each batch of 5 parallel artifact downloads.
This avoids GitHub secondary rate limit errors when downloading ~1000
test report artifacts in the slack reporting job.
Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>