refactor: hpc.env_vars() is now a function; skip exporting sing cache

[kelly-sovacool]

Changes

do not export a singularity cachedir by default. it gets overridden by the nextflow config anyway, making troubleshooting more confusing than it needs to be.

Issues

resolves #212

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 73.51%. Comparing base (eedf42a) to head (3b8e8e1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #222      +/-   ##
==========================================
+ Coverage   73.42%   73.51%   +0.08%     
==========================================
  Files          28       28              
  Lines        1938     1948      +10     
==========================================
+ Hits         1423     1432       +9     
- Misses        515      516       +1     

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