[fix] formatting cmdline for shelltask

[djarecka]

Acknowledgment

• I acknowledge that this contribution will be available under the Apache 2 license.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Summary

the formatting didn't work well for inputs with sep field and argstr containing f-string, tests added

Checklist

• All tests passing
• I have added tests to cover my changes
• I have updated documentation (if necessary)
• My code follows the code style of this project
  (we are using black: you can pip install pre-commit,
  run pre-commit install in the pydra directory
  and black will be run automatically with each commit)

[codecov]

Codecov Report

Merging #466 (9176136) into master (bc71076) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #466   +/-   ##
=======================================
  Coverage   82.59%   82.59%           
=======================================
  Files          20       20           
  Lines        3975     3976    +1     
  Branches     1094     1094           
=======================================
+ Hits         3283     3284    +1     
  Misses        494      494           
  Partials      198      198           

Flag	Coverage Δ
unittests	82.52% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pydra/engine/task.py	86.80% <100.00%> (+0.04%)	⬆️

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