Install script for newer version of Haxe on Linux

The recommended way to install Haxe on Linux is to use the official packages, but these do not support any beta or RC releases.

I gave up building from source and updated my old Linux install script:

(The current script is linked to the final dev build on the day of the Haxe 4.0.0-rc.1 release)

The HAXE_VERSION line can be updated using a newer or different nightly build name