Ensure PHP installed by EE is used after installation

> I think this problem is occurring when the user already has PHP installed. At that time, even if EasyEngine installs PHP, it is not being used. Default PHP of user is being used which may not have required PHP extensions (pcntl, curl, sqlite3, zip). I think we need to use update-alternatives to ensure that our version of PHP is being used.

Quoting my above comment for visibility from https://github.com/EasyEngine/easyengine/issues/1403#issuecomment-717189448.

There are few solutions,

1. Use `update-alternatives` to update system-wide PHP.
2. Update [starter script](https://github.com/EasyEngine/easyengine/blob/develop/bin/ee#L31) to prefer our EE version(7.3 now) if available, else use whichever PHP is present.