fix: lowercase SIFs dir

[kelly-sovacool]

Changes

Use SIFs instead of SIFS everywhere.

Also make singularity_sif_dir a dynamic property of the HPC Cluster class to avoid code duplication.

Issues

see CCBR/RENEE#181

PR Checklist

(Strikethrough any points that are not applicable.)

• This comment contains a description of changes with justifications, with any relevant issues linked.
• Write unit tests for any new features, bug fixes, or other code changes.
• [ ] 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/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 32.41%. Comparing base (c338de2) to head (56c2cbd).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #23      +/-   ##
==========================================
+ Coverage   31.56%   32.41%   +0.84%     
==========================================
  Files          17       17              
  Lines        1264     1265       +1     
==========================================
+ Hits          399      410      +11     
+ Misses        865      855      -10     

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