pass BOOST_JAM_TOOLSET to calling script

A note in bootstrap.bat file mentioned that the toolset should really
be retrieved from build.bat, but the developer didn't know how.

The comment I'm referring to is here: https://github.com/boostorg/boost/blob/bd86bb8943e01c266e5c2882706f8301e9a752b7/bootstrap.bat#L34

I'm not sure if that's still relevant or not, since I noticed the boostrap.bat file in this module of the project doesn't work the same.