boost: fix the bjam build for Cray (#10295)

Fixes issues #9613 and #3209.

Remove the spack wrapper directories from PATH for the bootstrap step.
This was breaking the build for Cray (and other cross-compile) because
bjam was built for the BE and died on SIGILL on the FE.

This only affects building bjam.  The boost libraries are still built
the normal spack way with the spack wrappers.