fix: ensure MPI & Intel libs found during pip install (keep env vars)

[aidancrilly]

Just getting starting with PolyChord but I had an issue with installation via python setup.py install on a cluster (Imperial College).

The build environment didn't carry over the necessary library and mpi paths to allow compilation.

Without I_MPI_ROOT, the build would fail on the include mpif.h in utils.F90 as it can't find mpif.h

Without library paths, build would fail at the linking stage as it couldn't find libimf.so

Following additions to setup.py allowed compilation.

Software version info:
The modules loaded where:

Currently Loaded Modulefiles:
 1) tools/prod            3) zlib/1.2.13-GCCcore-13.2.0 <aL>     5) intel-compilers/2023.2.1             7) UCX/1.15.0-GCCcore-13.2.0 <aL>           
 2) GCCcore/13.2.0 <aL>   4) binutils/2.40-GCCcore-13.2.0 <aL>   6) numactl/2.0.16-GCCcore-13.2.0 <aL>   8) impi/2021.10.0-intel-compilers-2023.2.1  

Key:
<module-tag>  <aL>=auto-loaded  

which mpiifort returns:

/sw-eb/software/impi/2021.10.0-intel-compilers-2023.2.1/mpi/2021.10.0/bin/mpiifort