Save a hash of the hls.js release

[bouk]

This ensures the downloaded hls.js matches exactly and removes a dependency on cdn.jsdelivr.net

It also makes it easier for the nixpkgs version of mediamtx to get the hls.min.js updates, because right now this needs to be done manually

[aler9]

This has been merged, thanks.

Regarding the NixOS problem, in my opinion it has been fixed in the wrong way, since the proper solution consists into adding go generate to the package building process. Command go generate was introduced in order to run any pre-compilation step by using a single, standard, command. If pre-compilation steps are instead embed into the package and run manually, there's no guarantee that the software will compile correctly in the future.