fix: run choosenim update stable before downloading nim

[ganderzz]

Resolves #264

Run choosenim update stable before downloading and installing nim. This prevents a local stable pointing to an old version of nim.`

[dom96]

So I think you need it in both branches (one with -y and one without). I'd merge this but this needs testing, can you test it with choosenim not installed + choosenim installed and include the output you get? :)

[ganderzz]

@dom96 Updated. :)

With choosenim installed

Without