collect-assets could use cache checking to skip downloading assets that haven't been updated

[sjd78]

Specifically to help out developers, the collect-assets.js scripts could use basic caching HTTP headers to be able to skip downloading of any release assets (or workflow artifacts). If an asset hasn't been updated since the last download, skip the download.

ETag/If-None-Match headers could be used. The ETag would need to be stored and accessed in future (from our collect.json maybe?).

Last-Modified/If-Modified-Since request header on the asset fetch could be used. The If-Modified-Since header could be generated from the mtime of the previously downloaded file.

In either case, download could always be forced by removing the download_assets folder and running the script again.