Ensure PHP installed by EE is used after installation

[kirtangajjar]

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 EasyEngine/easyengine#1403 (comment).

There are few solutions,

1. Use update-alternatives to update system-wide PHP.
2. Update starter script to prefer our EE version(7.3 now) if available, else use whichever PHP is present.