Yank jxcore build defs

[jasonkarns]

I'd like to yank the jxcore build defs. (I don't believe they even build successfully anymore with most assets having gone offline.)

Before we do that, I'd like to create a separate nodenv plugin as a repository of "archived" build definitions. That way people who would like access to the old build defs (for whatever reason) can do so through the extra plugin.