Add optimized Spack recipe for OpenFOAM on hpc6a

[mhuguesaws]

Install Spack v0.20.x

 export SPACK_ROOT=/shared/spack
 git clone -c feature.manyFiles=true -b releases/v0.20 https://github.com/spack/spack $SPACK_ROOT
 echo "export SPACK_ROOT=$SPACK_ROOT" >> $HOME/.bashrc
 echo "source \$SPACK_ROOT/share/spack/setup-env.sh" >> $HOME/.bashrc
 source $HOME/.bashrc

Add Spack cache

spack mirror add v0.20.1 https://binaries.spack.io/v0.20.1
spack buildcache keys --install --trust

Install OpenFOAM 2206

spack compiler find
spack install openfoam@2206