GH Actions: fix download URL for box

[jrfnl]

Looks like the box project has moved organisations, so let's update the URL used to download the PHAR.

As GitHub will automatically redirect moved URLs anyway, I'm only pulling this to the develop branch.

[grogy]

Thank you, yes, project was moved

$  curl -I https://github.com/humbug/box
HTTP/2 301 
server: GitHub.com
...
location: https://github.com/box-project/box
...