ROOT: set LD_LIBRARY_PATH when ~rpath (#16899)
ROOT: set LD_LIBRARY_PATH when ~rpath (#16899)

Allow Spack to build with ROOT as an external dependency by setting
LD_LIBRARY_PATH: given that the external package was not built by
Spack, dependents would not be able to locate libraries using RPATHs
when running ROOT binaries.
LLNL/spack,iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack