fix: properly flag if running on Linux or Mac OSs

[G-Rath]

The OSUnix value is only meant to be used by plugins to indicate they're fine working with either Linux or macOS, but the OS still needs to be one of those values when scanning.

This is not an issue currently because we're just using extractors, but there are a few detectors that require a Unix-like OS

[G-Rath]

I'm pretty sure its safe to return this rather than error, though (assuming we don't actually want to error) it could be nice if scalibr had a plugin.OSUnknown either as a new value or an alias for this one, since that's what we want to represent here

[another-rex]

Hmm, can this log a warning that OS "xyz" is not officially supported.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 73.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 66.08%. Comparing base (6e5607b) to head (f314ff9).

Files with missing lines	Patch %	Lines
pkg/osvscanner/scan.go	73.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2051      +/-   ##
==========================================
- Coverage   66.09%   66.08%   -0.01%     
==========================================
  Files         172      172              
  Lines       16214    16225      +11     
==========================================
+ Hits        10716    10723       +7     
- Misses       4848     4852       +4     
  Partials      650      650              

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[G-Rath]

@another-rex I found this while trying to come up with tests for the detectors - afa06df shows that this is broken.

I've opened this so I don't forget, but I think it's probably best if we first get the snapshot failures on #2021 sorted before we dig into everything else including this

[another-rex]

Talked with other Scalibr folks, I think we're happy to add a OSUnknown in scalibr that just only matches OSAny.