Update CPATH in setup_environment for Eigen, so that the
Spack-generated module for Eigen will help builds outside of Spack
use the appropriate include prefix for Eigen headers
(<install_prefix>/include/eigen3/ rather than <install_prefix>/include/)
Note that this only updates the run-time environment, rather than the
build-time environment, so Spack builds depending on Eigen that use
pkgconfig will not be confused by the presence of the Eigen include
directory in CPATH.
0e22a94f81