Fetch copilot-language-server files from the official sources

[uiii]

Hi, I just found your plugin and it seems promising. However for my safety I tried to analyze your code with Github Copilot and check if it is legit 🤔 Copilot says it is more or less safe 😁

However I was curious when I saw you fetch some binaries from another repo (copilot.vim). I realized it is an official repo for vim integration by github so it's quite ok. But still I think you can fetch the language server files from the official repo for that https://github.com/github/copilot-language-server-release/releases to make it less suspicious.