Nim version 2.0 RC1

[ringabout]

It seems that choosenim only builds Nim with `csourcesUrl = 
"https://github.com/nim-lang/csources"`.

<https://cs.github.com/dom96/choosenim/blob/d929b737ef6b042a946c88a5fd2cdee64af6c292/src/choosenimpkg/download.nim#L18>