fix: set singularity cachedir inside workdir

[kelly-sovacool]

Changes

Set the singularity cachedir to /data/$USER/.singularity or $WORKDIR/.singularity if --singcache isn't provided.

Later we will provide a more robust solution that copies previously downloaded SIFs, after CCBR/CCBR_SnakemakeTemplate#1

Issues

fix #33

PR Checklist

(Strikethrough any points that are not applicable.)

• This comment contains a description of changes with justifications, with any relevant issues linked.
• Update docs if there are any API changes.
• Update CHANGELOG.md with a short description of any user-facing changes and reference the PR number. Guidelines: https://keepachangelog.com/en/1.1.0/

[kelly-sovacool]

test in progress, job ID: 35634872

[kelly-sovacool]

test in progress, job ID: 35634872

test was successful. @kopardev this is ready for your review.