Improve --parallel --singularity

[mr-c]

Issues found while running https://github.com/alpha-unito/cwl-montage-workflow on a system with 192 hyperthreads (2 x AMD EPYC 9454 48-Core Processors)

[codecov]

Codecov Report

❌ Patch coverage is 69.14894% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.11%. Comparing base (ef68f07) to head (85247bc).

Files with missing lines	Patch %	Lines
cwltool/singularity.py	68.81%	18 Missing and 11 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2187      +/-   ##
==========================================
- Coverage   85.12%   85.11%   -0.01%     
==========================================
  Files          46       46              
  Lines        8368     8405      +37     
  Branches     1956     1967      +11     
==========================================
+ Hits         7123     7154      +31     
- Misses        779      784       +5     
- Partials      466      467       +1     

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