When building Spack packages with Jenkins, please use the bb5 executors.
Then you will be able to install software with:
$ module load unstable git
$ git clone -c feature.manyFiles=true https://github.com/BlueBrain/spack.git
$ . spack/share/spack/setup-env.sh
$ mkdir fake_home
$ export HOME=${PWD}/fake_home
$ mkdir -p ~/.spack
$ ln -s /gpfs/bbp.cscs.ch/apps/hpc/jenkins/config/*.yaml ~/.spack
$ export SPACK_INSTALL_PREFIX=${HOME}/software
$ spack build-dev <my_package>
Note that a custom home directory is created to avoid any interference from a shared configuration of Spack.