* guix/scripts/pack.scm (singularity-environment-file): New procedure.
(squashfs-image): Use it, and create /.singularity/env/90-environment.sh.
* gnu/tests/singularity.scm (run-singularity-test)["singularity run,
with environment"]: New test, currently skipped.
* gnu/tests/singularity.scm (build-tarball&run-singularity-test): Add
GUILE-JSON to the profile.