Avoid /dev/null for spack@develop

[simonpintarelli]

Current spack@develop wants to read config from SPACK_USER_CONFIG_PATH despite it has been set to /dev/null. Use $HOME hidden by tmpfs instead.

[daint][simonpi@nid006880 build-bwrap]$ env --ignore-environment http_proxy="$http_proxy" https_proxy="$https_proxy" no_proxy="$no_proxy" PATH=/usr/bin:/bin:`pwd`/spack/bin HOME=$HOME make store.squashfs -j32
/dev/shm/simonpi/build-bwrap/bwrap-mutable-root.sh --tmpfs ~ --bind /dev/shm/simonpi/build-bwrap/tmp /tmp --bind /dev/shm/simonpi/build-bwrap/store /user-environment spack --version > spack-version
spack arch... ==> Error: Path is not a file or is not readable: /dev/null/config.yaml: [Errno 20] Not a directory: '/dev/null/config.yaml'

spack version... 0.23.0.dev0 (0fee2c234ef1eda5483a2da5c6bd679c48d7ab26)
checking if spack concretizer works...  failed, see /dev/shm/simonpi/build-bwrap/spack-bootstrap-output
make: *** [Makefile:15: spack-setup] Error 1

[bcumming]

looks good!