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]

In general the right idea, but see my comments. Thank you for working on this!

[dom96]

This can prompt, so I think it should go in the if branches below.

[dom96]

Also, you can override the chosen channel as you can see below, you cannot assume stable.

[ganderzz]

Gotcha. Updated. Not sure I totally got the first comment, so let me know and I'll get things updated!

[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

[dom96]

Sorry, but thinking about this some more I think this logic should be implemented inside choosenim. i.e. if choosenim stable is ran with the --firstInstall flag then run update implicitly. That way it's easier to keep this logic sound.