Account for non-UNIX platforms in sys.platform checks

[joshuacwnewton]

Otherwise, we'd get a "referenced before assignment" error on
Windows.

Checklist

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Description

This PR fills in the gaps for platforms outside of darwin and linux.

I think this check probably could use a more thorough rewrite, though? I'm skeptical of its purpose.

Linked issues

Fixes #3709.

[codecov]

Codecov Report

Merging #3722 (9163f28) into master (2c2a662) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

Flag	Coverage Δ
api-tests	22.30% <0.00%> (-2.16%)	⬇️
cli-tests	58.74% <0.00%> (+1.80%)	⬆️

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

Impacted Files	Coverage Δ
spinalcordtoolbox/scripts/sct_run_batch.py	70.95% <0.00%> (-0.90%)	⬇️

[mguaypaq]

LGTM

This check seems to be purely for printing diagnostic information to the logs, so it seems fine.